Submitted to Indoor Air 15 June 2020

Transmission of SARS-CoV-2 by
inhalation of respiratory aerosol in the
Skagit Valley Chorale superspreading
event
Shelly L. Miller1, William W Nazaroff2, Jose L. Jimenez3, Atze Boerstra4, Giorgio Buonanno5,
Stephanie J. Dancer6, Jarek Kurnitski7, Linsey C. Marr8, Lidia Morawska9, Catherine Noakes10
1

Mechanical Engineering, University of Colorado Boulder
Civil and Environmental Engineering, University of California, Berkeley, CA, USA
3
Dept. of Chemistry and CIRES, University of Colorado, Boulder, CO, USA
4
REHVA (Federation of European Heating, Ventilation and Air Conditioning Associations), BBA
Binnenmilieu, The Netherlands
5
Department if Civil and Mechanical Engineering, University of Cassino and Southern Lazio,
Cassino, Italy
6
Edinburgh Napier University and NHS Lanarkshire, Scotland
7
REHVA Technology and Research Committee, Tallinn University of Technology, Estonia
8
Civil and Environmental Engineering, Virginia Tech, Blacksburg, VA, USA
9
International Laboratory for Air Quality and Heath (ILAQH), WHO Collaborating Centre for
Air Quality and Health, School of Earth and Atmospheric Sciences, Queensland University of
Technology, Brisbane, Queensland, Australia
10
School of Civil Engineering, University of Leeds, United Kingdom
2

1

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Abstract
During the 2020 COVID-19 pandemic, an outbreak occurred following attendance of a
symptomatic index case at a weekly rehearsal on 10 March of the Skagit Valley Chorale (SVC).
After that rehearsal, 53 members of the SVC among 61 in attendance were confirmed or strongly
suspected to have contracted COVID-19 and two died. Transmission by the aerosol route is
likely; it appears unlikely that either fomite or ballistic droplet transmission could explain a
substantial fraction of the cases. It is vital to identify features of cases such as this to better
understand the factors that promote superspreading events. Based on a conditional assumption
that transmission during this outbreak was dominated by inhalation of respiratory aerosol
generated by one index case, we use the available evidence to infer the emission rate of aerosol
infectious quanta. We explore how the risk of infection would vary with several influential
factors: ventilation rate, duration of event, and deposition onto surfaces. The results indicate a
best-estimate emission rate of 970 ± 390 quanta h-1. Infection risk would be reduced by a factor
of two by increasing the aerosol loss rate to 5 h-1 and shortening the event duration from 2.5 to 1
h.
Keywords: aerosol transmission, infectious disease, ventilation, virus, pandemic, risk

Practical Implications
During respiratory disease pandemics, group singing indoors should be carefully
managed as singing can generate large amounts of aerosolized virus if any of the singers
is infected.
 Ventilation requirements for spaces that are used for singing (e.g., buildings for religious
services and rehearsal/performance) should be reconsidered in light of the potential for
aerosol transmission of infectious diseases.
● Systems that combine the functions heating and ventilation (or cooling and ventilation)
should be accompanied with a disclaimer saying “do not shut this system off when people
are using the room; turning off the system will also shut down outdoor air supply, which
can lead to the spread of airborne infections.”


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Introduction
SARS-CoV-2 was first reported in China at the end of 2019 and rapidly spread to the rest of
the world over the subsequent months. Evidence from laboratory studies has shown that the
SARS-CoV-2 virus can remain infectious while airborne for extended periods.1,2 The virus has
been detected by PCR in the air in several healthcare environments.3-9 Researchers have reported
values for the SARS-CoV-2 viral load in the mouth that span an extraordinarily broad range:
from 102 to 1011 copies per mL of respiratory fluid.10-12 Viral loads vary over the course of the
disease, tending to peak near the onset of symptoms.
Aerosol transmission is now strongly suspected to play a significant role in superspreading
events (SSEs) under certain conditions. SSEs occur when a large number of secondary
transmissions are produced early in an outbreak and transmission is sustained in later stages.13
Some people release respiratory aerosol at an order of magnitude greater rate than their peers and
might contribute to superspreading events.14 The very broad range of viral loads in respiratory
fluids may also be an important factor influencing SSE. An infectious respiratory aerosol is a
collection of pathogen-laden particles in air emitted during respiring activities of an infected
individual. 15
Conditions that promote outbreaks of COVID-19 include large indoor gatherings and poor
ventilation. An assessment of over 300 COVID-19 outbreaks involving three or more people
confirmed that all transmission could be linked to indoor environments.16 A Japanese study based
on contact tracing concluded that the odds that a primary case transmitted COVID-19 in a closed
environment was 18.7 times greater compared to an open-air environment. 17 Another study from
Japan concluded that the typical settings for superspreading events included “singing at karaoke
parties, cheering at clubs, having conversations in bars, and exercising in gymnasiums”.18 Poor
ventilation has been a driving factor in other infectious disease outbreaks. Transmission of
influenza from one index case to 72% of the onboard passengers occurred on an airplane that
was grounded for three hours, during which the ventilation system was inoperative.19
In this paper, we first discuss the outbreak and establish the likelihood of an important
contribution from aerosol transmission. We then estimate the infectious quanta emission rate
during a choir rehearsal that has been identified as a superspreading event. Quanta are used to
represent infectious respiratory aerosol when the actual viral dose in the aerosol and the human
dose-response required to cause infection are unknown.17,21 We then explore the sensitivity of the
secondary attack rate of infection to the loss rate of aerosolized virus, whether by ventilation,
deposition onto surface, or biological decay, as well as to duration of the event.

Case Study
A SSE occurred in Skagit Valley, Washington, USA.22,23 When the Skagit Valley Chorale
(SVC) met on the evening of March 10, 2020, one person attending the rehearsal had cold-like
symptoms that had developed three days earlier; that individual subsequently tested positive for
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COVID-19. This person is considered the “index case.” Within a few days of the news report, we
contacted the journalist that reported on the event, the county health department, and a member
of the choir. An initial questionnaire and multiple follow up questions were sent to the choir
contact, and most of the choir members provided information on their activities. The responses
are included in the Supplementary Information in the same form as provided to us, except for
items removed due to privacy concerns. At the time of the rehearsal the Skagit County Health
Department was not recommending widespread closure of public venues or public events. They
were recommending that those 60 y of age and older, or persons with underlying medical
conditions, should avoid large public gatherings. Choral members were told to not attend on
March 10 if they were sick with any kind of symptoms or if they had concerns.
The chorale met in the Fellowship Hall of a church in Mount Vernon, Skagit County. The day
after the rehearsal on March 11, the governor of Washington recommended physical distancing
and no large group meetings in three other nearby counties. At the time of the rehearsal, there
were no known COVID-19 cases in Skagit County, nor were any closures in effect. Before
detecting the cluster on March 17, Skagit County had developed seven COVID-19 cases. The
likelihood of a second index case at the rehearsal can be estimated as 0.32%, from the seven
other cases that had been reported in the county at the time in which the choir outbreak became
known, given the population of the county, and an assumption of 50% asymptomatic cases.
Asymptomatic cases were less likely than for an average population given that the choir
members are more elderly than the general population; e.g. 75.5% of those who became ill were
65 or more years old.22
The SVC has 122 members, but only 61 attended rehearsal on March 10, amid concerns
about COVID-19 transmission. Precautions were taken during rehearsal, including the use of
hand sanitizer, no hugging and no handshakes.23 All 120 chairs were arranged by 3 people who
arrived early, and members sat in their usual chairs, which increased their distance compared to
other well-attended rehearsals. Lateral distance between chair centers (and thus nose / mouth
distances) was ~0.75 m, while forward distance between rows was ~1.4 m.
Some members began experiencing illness from March 11 to March 22. The timing of these
potential secondary infections is consistent with what is known about the temporal dynamics of
virus shedding and serial interval for COVID-19.24 In particular three cases were identified the
day after the rehearsal, according to the county report22, although the choir members are not
aware of cases before March 12 (see Supplemental Information (SI)). Several published analyses
of the incubation time of COVID-19 include some probability of developing symptoms within
one day of infection.25,26, 27, 28 This would be more likely in the case of receiving a high viral dose,
as would be expected in an event like this with such a high attack rate. It is also possible that
there was more than one index case present at the rehearsal, and the impact of this possibility on
our analyses is discussed below.
Among the 61 attendees at the rehearsal, 53 cases in total were subsequently identified
including the index case, with 33 confirmed through positive COVID-19 tests and 20
unconfirmed but probable secondary cases based on symptoms and timing. Testing was

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unavailable to many of the choir members (see SI). Accounting for the one presumed index case,
the secondary infection attack rate is thus in the range 32/60 to 52/60, or 53-87%.
The large number of infections arising from this event, compared to the low incidence in the
county at the time, makes it unlikely that they were acquired at a different setting than the choir.
This inference is consistent with the conclusion of the county health investigators, who
interviewed all choir members and investigated other possible avenues for contact: transmission
most likely occurred at the March 10th rehearsal.22 It is also consistent with the opinion of the
choir members (see SI). Infection of many choir members at the prior March 3rd rehearsal is also
very unlikely, as discussed by Hamner et al.22 Given a median incubation time of 4-5 days, if
multiple members had been infected at the March 3rd practice, 70-80% of them should have
presented symptoms by the time of the March 10th practice.27
A seating chart obtained through personal communication showed the layout of participants
among 120 chairs plus the position of the choir director and piano accompanist. Although the
chart cannot be reproduced because of privacy concerns,22 a centrally important point for
interpreting the cause of transmission is that the cases occurred throughout the room with no
clear spatial pattern. Specifically, dividing the 120 seats into quadrants of 30 seats each, the
occupancy levels (seats occupied/seats available) span a narrow range of 44-53%. The infection
rate (reported infections/seats occupied) are substantial in each quadrant, with a low of 43% and
being in the range 71-87% in the other three quadrants.
The rehearsal started at 6:30 pm. The SVC rehearsed in a single group in the Fellowship Hall
for 45 minutes, with the members in fixed positions in their usual seats, then split into two
approximately equal-sized groups for 45 minutes. One group, mostly male singers, went to
practice in a different room of the church, where the singers sat in the church pews and the piano
was off to the side. A second group stayed in the Fellowship Hall. The attack rate was higher in
the group that stayed in the Fellowship Hall (see SI), which is consistent with the presence of the
index case in that room during the separate rehearsals. Transitions between the 3 phases of
rehearsal were rapid (see SI). After practicing separately, and following a 10-minute break, the
members reconvened in the Fellowship Hall for another 50 minutes, until 9 pm. During the split
session, those who remained in the Fellowship Hall occupied about half of the space, and thus
had a similar person density as during the whole-group rehearsal (see SI).
Limited information is available about the heating and ventilating system; what was learned
from personal communications is summarized here. The Fellowship Hall is heated and ventilated
with a mechanical air heating system including an outdoor air intake and air recirculation. The
air handling unit has a relatively new commercial forced-air furnace (see SI for the system
capacity details). The furnace is installed with an outside make-up air function and it also has a
separate combustion air intake, which is standard for gas appliances. But it is not known how
much outside make-up air was supplied to the building that evening. The furnace is also outfitted
with a MERV 11 filter, which has a rated single-pass efficiency of ≥ 30-65% for aerosol particles
of diameter 1 µm or larger.29,30 Three supply air registers are situated 2.4 m above the floor on
one wall with a single return on an adjacent wall, just above the floor (~0.15 m). Someone in the
front office reportedly turned on the heating system prior to the rehearsal to warm the space, and
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the thermostat was set to 20 °C (68 °F). It was about 7 °C (45 °F) outside, so the heating was on
at the start of the rehearsal, but with so many people in the room, it did not need to stay on to
maintain a comfortable temperature. During the entire rehearsal no exterior doors were open. It is
not known whether the forced-air furnace fan operated (only) under thermostatic control or
whether it ran continuously.

Evidence Related to the Routes of Transmission
There are considered to be three primary routes of transmission for COVID-19: (1) direct
contact (e.g. shaking hands) or indirect contact with contaminated objects (“fomites”), followed
by touching one’s eyes, nostrils, or mouth; (2) large ballistic droplets that travel directly from an
infected person’s nose or mouth to a susceptible person’s eyes, nostrils, or mouth; (3) exhaled
respiratory aerosols, which can linger in the air for minutes to hours, and may infect by
inhalation.
There is no specific evidence that COVID-19 is transmitted via the fomite route,31 and the US
CDC has stated that while possible, this route is considered less likely,32 possibly because of
rapid inactivation demonstrated for lipid-enveloped viruses on human skin.33,34,35,36 At the time of
the chorale rehearsal on 10 March 2020, because of emerging concern about SARS-CoV-2,
person-to-person contact and touching of surfaces was consciously limited, and hand sanitizer
was used. No one reported direct physical contact between attendees to the County Public Health
investigators.22 Although some choir members helped arrange the chairs and ate snacks during
the intermission, the index case did not participate in these activities, and many other members
reported not eating the snacks.22 Thus fomite transmission from the index case via chairs or
snacks can be excluded. The index case used one of the bathrooms during the event and thus
touched the door handle and other surfaces there, but only about six other choir members used
that restroom (see SI), and many choir members who did not use any of the restrooms were also
infected. Indeed, the clustering of infected cases on the seating chart does not support
transmission from a point surface contact(s) unless the people who sat together all touched the
same contaminated surface. Thus, it appears highly improbable that the direct and indirect
contact routes could account for a significant fraction of the transmission during this event.
There is no direct evidence of transmission by ballistic droplets for any disease in the
literature.37 The risk of widespread transmission owing to large ballistic droplets during close
proximity situations would seem to be low in this event, considering that it is likely to have been
only one index case, who was seated in close proximity to only a small proportion of the other
chorale members. One half of the chairs were unoccupied, increasing the distance between
members. No one was located within 3 meters in front of of the index case (where respiratory
droplets from the index case would be expected to have landed (See SI) during either of the
rehearsal periods. Two other members where located within 1 m to each side of the index case
during parts of the rehearsal, while four other members were located within 2 m behind (and one
2 m to the side) of the index case during parts of the rehearsal. Ballistic droplets, propelled
forward by exhaled breath, could not have traveled backwards in this low ventilation situation.
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There was a single 10-min break (see SI), during which the participants talked with each
other, mostly in groups of 3-4 people, while the index case conversed minimally with others
throughout the rehearsal and the break (see SI). Many members arrived shortly before and left
immediately after the practice (see Ref. 19 and SI). However, about 15 minutes of close
proximity is thought to be needed for transmission.32 Thus, it is physically not possible for the
index case (or even several index cases) to have conversed with and impacted ballistic droplets
onto 53 other members in such a short time.
Literature evidence suggests that singing could have been a contributing factor to the high
secondary attack rate compared to other common indoor activities. The rate of aerosol emission
during vocal activities increases with voice loudness.15 A study of respiratory emissions also
found higher emission rates of respiratory droplets to be associated with more extensive
vocalization.38 Outbreaks of tuberculosis, a disease known to be transmitted via inhalation, have
been linked to singing.39-41 At the time this article is being written, there have been additional
media reports of COVID-19 outbreaks associated with choirs. Cases with high secondary attack
rates have been reported in the Netherlands, Austria, Canada, Germany, England, South Korea,
Spain, and France.42,43,44
Loudon and Roberts45 characterized respiratory aerosol emitted during talking, singing and
coughing. They reported that “fewer droplets were expelled during singing than during talking,
but a higher proportion of them were in the smaller size range. The percentage of droplets still
airborne as droplet nuclei after a 30-minute settling period were 35.7, 6.4, and 48.9 for singing,
talking, and coughing, respectively.”
If transmission by fomites and/or ballistic droplets were the dominant modes of transmission,
then the secondary attack rate should have been much smaller than the observed range of 5387%. We would also expect to see the secondary cases predominantly among those in closer
proximity to the index case rather than distributed throughout the rehearsal room. Even in the
case that a second index case had been present, the same considerations make such wide
transmission by the fomite and ballistic drop routes very unlikely to explain the observed very
high attack rate. Per Occam’s razor, this explanation seems most probable: that inhalation of
infectious respiratory aerosol from “shared air” was the leading mode of transmission.

Modeling Aerosol Infection Risk
This distinctive superspreading event, occurring in an enclosed community facility, with
indoor space shared for a specified period of time, offers a good opportunity to examine a range
of physical parameters that influence the eventual outcome. Our analysis was undertaken to
explore whether this outbreak could have happened due to aerosol transmission and how future
outbreaks could be avoided. In assuming only aerosol transmission what follows represents a
worst-case scenario for aerosol transmission, i.e., highest possible quanta generation rate from
the event. If a few cases arose from fomite or ballistic droplet routes, the quanta emission rate of
the aerosol route would be proportionally lower.

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There is no evidence to suggest that more than one person was infected and showing
symptoms at the time of the rehearsal. Asymptomatic transmission, however, is important in the
spread of COVID-19.24 Available evidence suggests that 50% of transmission happens while
asymptomatic; it is, however, estimated that only 20% of cases remain asymptomatic.47,48,49 Thus,
it is possible that individuals at the rehearsal were asymptomatic transmitters; however, as
described earlier, we estimate this probability to be very small because of the low community
prevalence of COVID-19 at the time of the rehearsal. Assuming that there is only one index case
to account for all transmission and that all transmission was through aerosol is a conservative
approach and provides a basis that can be used to develop precautionary mitigation approaches.
Hence, on the basis of the available information about this event, a modeling effort was
undertaken with two goals. The first goal was to estimate an average quanta emission rate that is
consistent with the evidence, while assuming that all transmission happens through exposure to
aerosol from a single index case. This calculation proceeds in two steps: determining the average
aerosol quanta concentration from the reported secondary infection attack rate, and then
evaluating the emission rate that would have produced the inferred average concentration. The
second modeling goal was to explore how a change in the loss rates, for example owing to
improved ventilation and filtration, would have altered the infection risk. In pursuing both goals,
the modeling effort uses an idealization of the more complex real situation, in part because some
key data are lacking. A similar approach has been used in other studies to explore aerosol
infection risk in indoor environments.20,50
The model of infection risk due to aerosol transmission is based on the Wells-Riley
formulation,46,51 as amended by Gammaitoni and Nucci.52 In applying this approach, these
assumptions are made: i) there is one infectious individual who emits SARS-CoV-2 quanta at a
constant rate throughout the event, ii) there is no prior source of quanta in the space, iii) the
latent period of the disease is longer than the time scale of the event, iv) the infectious respiratory
aerosol quickly becomes evenly distributed throughout the room air, and v) infectious quanta are
removed by first-order processes reflecting the sum of ventilation, filtration, deposition, and
airborne inactivation. The assumption that the indoor environment can be modeled as well-mixed
is substantiated in this case by the broad spatial distribution of secondary infections among the
rehearsal participants. Additional information is provided in the SI describing supporting
evidence of the well-mixed assumption. In epidemic modeling, where the aim is to assess the
disease spread in the community, it is impossible to specify geometries, ventilation efficiency,
and the locations of the infectious sources in each microenvironment. Therefore, adopting the
well-mixed assumption is generally more reasonable than hypothesizing about specific patterns
of emissions, airflow and removal processes.53
The modeled probability of infection (p) is related to the number of quanta inhaled (n)
according to equation (1):51
p=1−e−n

8

(1)

 Submitted to Indoor Air 15 June 2020
Equation (1) is used to estimate the average quanta concentration during the practice, given
estimates of the probability of infection based on the secondary attack rate. The aerosol quanta
concentration increases with time from an initial value of zero following a “one minus
exponential” form, which is the standard dynamic response of a well-mixed indoor volume to a
constant input source. The time-average quanta concentration (Cavg, q m-3) is the quanta inhaled
divided by the volume of air breathed. The volume of air breathed (m3) is equal to the duration of
the event (D, h) multiplied by the volumetric breathing rate of rehearsal participants (Qb, m3 h-1).
A well-mixed material balance model for the room (equation (2)) is applied next to relate the
quanta concentration, C (quanta per m3), to the emission rate, E (quanta per h):
dC E
= −λC
dt V

(2)

Here V = volume of the rehearsal hall (m3) and λ = first-order loss rate coefficient for quanta
(h-1) due to the summed effects of ventilation (λv), deposition onto surfaces (λdep), and virus decay
(k).54 Assuming the quanta concentration is 0 at the beginning of the rehearsal, equation (2) is
solved and the average concentration determined as follows:
C ( t )=

E
( 1−e−λt )
λV
D

(3)

[

1
E
1 (
−λD
C avg= ∫ C ( t ) dt=
1−
1−e )
D 0
λV
λD

]

(4)

Here, t = time (h). Equation (4) is rearranged to solve for the emission rate, E:

[

E= λV C avg 1−

1
( 1−e−λD )
λD

−1

]

(5)

A Monte Carlo simulation was run (N = 1000 iterations) to estimate E for the superspreading
event given a range of input values. The unknown parameters (p, Qb, λv, λdep, k) were specified as
probabilistic using uniform distributions bounded by specified upper and lower limits. These
parameters were assumed to be uncorrelated. A sensitivity analysis was undertaken to explore
how using different parametric distributions influenced the predictions. See SI for details and
results.
The ranges of the uncertain model parameter values explored in the primary Monte Carlo
simulation are summarized in Table 1. Constant values were used for the volume of the
Fellowship Hall and the rehearsal duration.
Table 1. Parametric Values used in the Monte Carlo Simulation for Estimating E
Parameter
Value(s)
Distribution
Reference(s)
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 Submitted to Indoor Air 15 June 2020
Probability of Infection, p (%)
Volumetric Breathing Rate,
Qb (m3 h-1)
Loss Rate due to Ventilation, λv
(h-1)
Loss Rate due to Deposition
onto Surfaces, λdep (h-1)
Loss Rate due to Virus
Inactivation, k (h-1)
Volume of Rehearsal Hall, V
(m3)
Duration of Rehearsal, D (h)

53-87
0.65-1.38

Uniform
Uniform

19
30, 31

0.3-1.0

Uniform

Appendix

0.3-1.5

Uniform

32, 33

0-0.63

Uniform

1, 2

810

Constant

2.5

Constant

Personal
Communication
19

The lower breathing rate used in our simulations was from Binazzi et al.56 who reported
volumetric inhalation rates of singers to be in the range 0.22-1.0 m3 h-1. The upper breathing rate
was from Adams et al.55 for light activity (walking female and male middle age adults).50 SARSCoV-2 was found in air samples in two size ranges: 0.5-1 µm and > 2.5 µm.7 The surface
deposition loss rate range was based on data from Thatcher et al.58 and Diapouli et al.57 The range
of values for virus decay is based on two sources: Fears et al.1 showed no decay in viruscontaining aerosol for 16 hours at 53% RH, whereas van Doremalen et al.2 estimated the half-life
of aerosol SARS-CoV-2 is 1.1 h, which equates to a decay rate of 0.63 h-1. The loss rate due to
ventilation is likely to have been in the range from 0.3 to 1 h-1 (see SI). We did not include
filtration in our estimation of the loss rate.

Results
The mean (± standard deviation) inferred emission rate was E = 970 ( 390) quanta per h.
Additional statistics for the distribution of E from the Monte Carlo simulation are as follows:
geometric mean = 900 q h-1; geometric standard deviation = 1.5; 10th, 25th, 50th, 75th and 90th
percentiles: 550, 680, 910, 1180, 1510 q h-1.
We reiterate that the emission rate was derived based on a base case assumption of one index
case and all transmission by aerosol inhalation. It is plausible that more than one person
attending the rehearsal was infectious. If this was the case then the inferred emission rate would
represent the sum of emission rates from each of the infectious individuals. The analysis assumes
that viral transmission all occurred through aerosol inhalation. If additional transmission routes
played some role in this outbreak, then our the actual quanta emission rates would have been
correspondingly lower.
Quanta emission rates for influenza have been reported to be in the range 15-128
quanta h-1;20,59 for measles: 5,580 q h-1;60 and for tuberculosis: 1.25 to 30,840 q h-1 (the high value
attributed to intubation).61 The quanta for SARS transmission in a hospital and in an elementary
school was estimated to be 28 q h-1.62 A forward model was used to estimate a large range of

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 Submitted to Indoor Air 15 June 2020
estimated quanta emission rates for SARS-COV-2, depending on activity level and respiratory
activity: 10.5-1030 quanta h-1.50
To explore the influence of changing the loss rate on the probability of infection, we
performed sensitivity simulations in which we varied the loss rate. In these simulations, we used
the mean emission rate of E = 970 q h-1 and a constant volumetric breathing rate of Qb = 1.0 m3 h1
. If λ is systematically increased by some combination of increased ventilation, deposition,
filtration, and inactivation loss rates, how would the probability of infection decrease? We also
explored what would happen if the emission rate was set at the 10th and 90th percentile values
from the Monte Carlo simulation. Using the model equations above with λ ranging from 0.6 to
12 h-1, the percentage of the rehearsal participants infected is determined. The results are plotted
in Figure 1.
E=550

E=970

E=1510

100
90

Probability of Infection %

80
70
60
50
40
30
20
10
0
0

2

4

6

8

10

12

Loss rate coefficient, � (per h)
Figure 1. Probability of infection for each rehearsal participant as a function of loss rates for
varying aerosol quanta emission rates (E, q h-1). Infection probability is plotted for the predicted
mean emission rate (970 q h-1) and the 10th and 90th percentile emission rates (550 and 1510 q h-

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1

, respectively.) Constant values were assumed for rehearsal duration (2.5 h), indoor volume
(810 m3) and volumetric breathing rate (1.0 m3 h-1).
A key point displayed in Figure 1 is that, for the mean value E = 970 q h-1, increasing the loss
rate coefficient from a nominal baseline value of 0.6 h-1 to 5 h-1 would reduce the probability of
infection by a factor greater than two, from 91% to 42%. For the full range of loss rates plotted
in Figure 1, the infection risks spans a factor of eight: from 98% to 13%.
We also explored how changing the duration of the event would impact the probability of
infection as a function of loss rate. Again, we use the mean emission rate of 970 q h-1 and a
volumetric breathing rate of 1.0 m3 h-1. For durations ranging from 0.5 to 2.5 hours, and total loss
rates ranging from 0.6 to 12 h-1, the predicted percentage infected spanned a broad extent, from
4% to 91%. The results are plotted in Figure 2.
D=0.5

D=1

D=2

D=2.5

100
90

Probability of Infection %

80
70
60
50
40
30
20
10
0
0

2

4

6

8

10

12

Loss rate coefficient, � (per h)
Figure 2. Probability of infection as a function of loss rates for varying event duration (D, h). A
mean emission rate (970 q h-1) and constant volumetric breathing rates of 1.0 m3 h-1 were
assumed.

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Discussion
The inferred emission rate of 970 quanta h-1 is plausible given observations of SARS-CoV-2
in aerosol samples collected in hospitals and exhaled breath condensate collected from patients.
Concentrations of viral RNA in patient rooms averaged 3000 ± 2700 gene copies m-3 across 18
measurements in Nebraska9 and 2600 ± 1000 gene copies m-3 across two measurements in
Singapore.3 If the dominant removal mechanism was ventilation at an average rate of 13 h-1 in
Nebraska and 12 h-1 in Singapore, then these concentrations correspond to emission rates of the
order 106 gene copies h-1 from a patient. This emission rate matches the range measured directly
in the exhaled breath condensate of patients.63 Evidence suggests that ratio of gene copies to
infectious virus is roughly 103,64,65 so the emission rate of 106 gene copies h-1 would correspond to
103 infectious virions emitted per hour. This compares favorably to the quanta emission rate if
the infectious dose is close to 1 plaque forming unit (PFU). We do not yet know the doseresponse relationship for SARS-CoV-2, but prior work indicates that the dose of SARS-CoV
corresponding to illness in 10% and 50% of those exposed is 43 and 280 PFU.21 (Watanabe et al.
2010) For influenza an inhaled dose as low as 0.7-3.5 PFU is sufficient to cause seroconversion
in 50% of subjects.66
The emission rate can also be estimated by combining evidence on respiratory aerosol in
exhaled breath with viral loads for SARS-CoV-2 in saliva. Concentrations of respiratory aerosol
in exhaled breath that are smaller than 10 µm diameter are in the approximate range 1-10 nL m-3
for vocalization activities.38 For this concentration range, a volumetric breathing rate of 1 m3 h-1
would produce an emission rate of 1-10 nL h-1 of respiratory aerosol. In limited sampling of
SARS-CoV-2 in saliva and other respiratory fluids, viral loads as high as 1011 gene copies mL-1
have been reported.10,11,12,67 At 10 nL h-1, a viral load in respiratory fluid of 1011 gene copies per
mL (= 105 gene copies nL-1) would lead to an emission rate of 106 gene copies h-1, similar to the
rate calculated above. Several factors contribute to substantial uncertainty in these comparisons
including variability in viral shedding with type of respiratory activity,68 viral load among
infectious persons69, the dose-response relationship, and other factors. However, the estimates do
support the plausibility of the inferred quantum emission rate.
This modeling analysis has explored the very probable situation in which transmission by
inhaling respiratory aerosol that was released during singing caused a large COVID-19 outbreak.
Accumulating evidence points to these factors being important for increasing the risk of aerosol
transmission indoors: dense occupancy, long duration, loud vocalization, and poor ventilation.
In the domain of indoor environmental quality control, the first and best measure is generally
to minimize indoor emissions.70 Because it is not yet possible throughout communities to
identify individuals who are highly infectious and therefore are potential superspreaders,
effective source control can not be so well practiced, short of suspending large gatherings of
high-risk indoor events. Risks would be reduced if fewer people attended, if durations were
shorter, and if attendees wore masks. The simulation results presented here show that the risk of
secondary infections can be substantially reduced although not practically eliminated through a
combination of increasing removal rates and by limiting the duration of indoor activities. The
13

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high ventilation rate in the hospital settings combined with other controls such as use of isolation
rooms and effective personal protective equipment is likely to mitigate transmission from a high
viral shedder in the healthcare environment.3,9 In general community indoor spaces, which are
not dedicated to infection prevention, controlling aerosol diseases transmission remains a great
challenge during this pandemic. Ventilation rates corresponding to current standards would allow
occupancy duration of only about 0.5 h for an infection risk level below 10% for a such high
emission activity as investigated here. Indoor environmental quality control measures available
to improve conditions include enhanced ventilation, mechanical filtration, and germicidal
ultraviolet disinfection.71,72 Widespread application of effective indoor environment controls
could help limit the extent of superspreading events and therefore contribute to slowing the
pandemic spread.

Acknowledgements
We wholeheartedly thank Carolynn Comstock for acting as choir spokesperson, providing
extensive information on the details of the outbreak, and answering many questions. We also
thank the choir members for their extensive cooperation through the spokesperson, Richard Read
of the Los Angeles Times for his rapid response to our inquiry and detailed discussions, and Lea
Hamner and the rest of the Skagit County Public Health Dept for very useful information and
discussions about the event.

Conflict of Interest
The authors of this paper certify that they have NO affiliations with or involvement in any
organization or entity with any financial interest (such as honoraria; educational grants;
participation in speakers’ bureaus; membership, employment, consultancies, stock ownership, or
other equity interest; and expert testimony or patent-licensing arrangements), or non-financial
interest (such as personal or professional relationships, affiliations, knowledge or beliefs) in the
subject matter or materials discussed in this manuscript.

Data Sharing Policy
The data that support the findings of this study are available from the corresponding author
upon reasonable request.

14

 Submitted to Indoor Air 15 June 2020

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Supplemental Information
Ventilation Rate Estimates
The ventilation rate for the baseline case was estimated assuming that the HVAC fan was not
operating during the rehearsal and there was no recirculated air. We assume that once the
rehearsal space was heated prior to the practice, heating did not turn on again because the
metabolic energy generated by the SVC rehearsal attendees was sufficient to maintain a
comfortable temperature without supplemental heating. For conditions of metabolic activity at
1.2 met, clothing insulation of 1.0 clo, at 22 C (71 F), the metabolic heat generation per
occupant is 78 W. 1 Assuming that half of the metabolic energy goes to continuously heat the
room air (with the other half lost through the building envelope by conduction and to heat
storage), then each occupant would contribute 39 W to the ventilation air. Given the reported
difference between indoor and outdoor temperature (23 °F = 13 K) and the heat capacity of air (1
J g-1 K-1), one can derive the ventilation rate to be 39 W person-1 ÷ 13 J g-1 = 3 g s-1 person-1. At a
density of 1.2 g L-1, the resulting ventilation flow rate would be 2.5 L s-1 person-1. For a room
volume of 810 m3 with 61 occupants, the corresponding air-change rate would be 0.7 per h-1. We
bracket this estimate by applying an uncertainty of ± 50% so that the modeled range in the
Monte Carlo simulation is 0.3-1.0 h-1.
By way of comparison, we have estimated the outdoor air ventilation rate based on the
relevant ASHRAE standard combined with information from the mechanical drawings for the
rehearsal hall under the assumption that the HVAC fan was on for the event duration. The
outdoor make-up air flow specified by ASHRAE Standard 62.1 for places of worship (Table
6.2.2.1) is 2.5 L s-1 person-1 + 0.3 L s-1 per m2 of floor area.2 The default occupant density is 120
persons per 100 m2 of floor area. The corresponding outdoor air rate per m2 of floor area would
then be (120/100)  2.5 + 0.3 = 3.3 L s-1 m-2. The reported averaging ceiling height for the
Fellowship Hall is 4.5 m and the estimated floor area is 180 m2. The total ventilation flow rate
would therefore be 180  3.3 = 594 L s-1 = 2100 m3 h-1, corresponding to an air-change rate of 2.6
h-1. Additionally, mechanical drawings of the rehearsal hall show specifications of 3  1560 cfm
supply registers (indicated to be 8 ft above the floor along one wall). This information indicates
that the ventilation system is designed to supply 4700 cfm = 8000 m3 h-1, which would be a
mixture of outdoor air and recirculated indoor air (filtered through a MERV 11 filter). That
supply flow rate corresponds to 10 room volumes per hour. Applying the outdoor air flow rate
from ASHRAE 62.1, at this overall flow rate, the mix would be about 25% outside air and 75%
recirculated air. These 2.5-10 effective air changes per hour are unlikely to have been provided
during the rehearsal, based on personal communication received during our investigation.

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Assumption of Complete Mixing
A major assumption of complete mixing was used in this analysis. Many indoor spaces are
indeed appropriately modeled as well mixed and for this specific event, it is a reasonable
assumption due to the following considerations.
● There is an absence of any strong pattern in the spatial distribution of secondary cases,
which supports the well-mixed assumption.
● The thermal conditions during the rehearsal would tend to promote relatively rapid
mixing. In particular, we observe that the outdoor air temperature was quite cool (7 °C),
which would have led to a tendency for the wall and ceiling surfaces to be cool. The
metabolic heat release from occupants during the rehearsal is estimated to be about 60
persons x 80 W/person ~ 5 kW. The heat generated by distributed occupancy near floor
level would combine with cooling at the walls and ceiling to promote effective mixing so
as to generate relatively short mixing time scales.3

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● Although there is no perfect analog in the literature, the mixing study of Baughman et al.
(1995) is informative.4 In that study, use of a 500 W electrical heater in an unoccupied
31-m3 room generated a mixing time for a point-source pollutant release of 13-15 min.
Even faster mixing was attained in the case when heat was generated by incoming solar
radiation through a window onto the floor. These experimental findings document that
fairly fast mixing times can be realized purely through buoyancy-driven flow.
Taken in combination, these indications seem to us sufficient to substantiate our decision to
model this event using a well-mixed representation of the indoor space.

1

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Sensitivity Analysis on Parametric Distributions for the
Monte Carlo Simulation
To understand how the assumption that the model parameters were uniformly distributed, we
repeated the work reported in the main paper but using alternative distributions. We amended our
original Monte Carlo simulation in four separate variations. In each, one of the input parameters
is changed from what we reported in Table 1 to triangular or fixed-value distributions. These
four, respectively, address breathing rate, air-change rate, decay rate, and deposition rate.
 For breathing rate, the average for men and women age 60-70 was used from the U.S.
EPA’s Exposure Factors Handbook,5 selecting the “sedentary/passive activity” case for
the lower bound of the triangular distribution, and “moderate activity” case for the upper
bound. The range, then, is 0.34-1.4 m3 h-1 with an average, 0.87 m3 h-1, that is
moderately higher than would be the case for “light” activity. We applied both a
symmetric and asymmetric triangular distribution in separate simulations.
 For air-change rate, we used the earlier values of 0.3 and 1.0 per h as the bounds of a
symmetricaltriangular distribution.
 For decay, we used the two reported values from Table 1 as single, fixed options (0 and
0.63 per h).
 For deposition, the same conceptual approach as for air-change rate was used, with 0.3
and 1.5 per h selected as the bounding values in a symmetrical triangular distribution.
The results are summarized in Table S1.
Table S1. Monte Carlo Simulation Results using Different Parametric Distributions for
Breathing (Qb), Air-Exchange ( v), Decay (k), and Deposition ( dep) Rates.

Parameter
Qb (m3/h)
v (h-1)
k
k

1

Change
Triangular
Triangular
Fixed
Fixed

Min
0.34
0.3
0
0.63

Max
1.4
1
0
0.63

Quanta Emission Rate (E, quanta/h)
Mean ± St. Dev.
GM (GSD)
1200 ± 530
1100 (1.51)
1020 ± 410
940 (1.48)
870 ± 350
810 (1.48)
1100 ± 430
1030 (1.47)

These simulation results are indicative of the degree of uncertainty associated with unknown
input parameter distributions. Note that the mean estimated values of E for the five cases in
Table S1 range from about 10% less to about 20% more than the primary estimate of E = 970 (
390) quanta per h reported in the main paper.

1 Each input parameter is changed separately from the original base-case simulation.

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Information Provided by Choir Spokesperson
The choir spokesperson responded to multiple requests for information, always in writing.
All of the responses have been reproduced in this section, except where the information would
reveal the identity or gender of the index case. The seating chart and some response text have
been removed for this reason. Text edited for privacy is enclosed in square brackets [like this].
The responses have been reordered by topic for ease of reading.

General Questions
Question: How many people contributed to the questionnaire responses you provided? Is it
accurate to say "a majority of the members present at the March 10 rehearsal"?
Choir Spokesperson: Yes, we had most of the choir answer.
Question: What was the age of those that did not fall ill? Where they younger than those who
fall ill? Or similar ages?
Choir Spokesperson: This outbreak was no respecter of age. The age range of infected
people was 31 to 84. Our group’s median age is probably 65.
Question: Where those who did not fall ill tested? (either immediately after the event, or later
on through antibody testing). Otherwise, some of them could have been infected, but be
asymptomatic, correct?
Choir Spokesperson: There was no testing available for most of us. Some doctors in the
group managed to get tests, but others were told not to come get a test unless they were much
sicker. One woman was told she looked “too healthy” to get a test. My husband and I were told
we were presumed positive. We knew we had it because we carpool with a doctor and his wife,
and they got tested. Remember, this was March 10 and we live in a rural area.
Question: You provided some corrections for our description of the event after you read the
original manuscript, which were incorporated in the version submitted to the journal. Would you
be ok reviewing a draft of the revised paper before resubmission, or specifically the sections
where we describe the event? So that we avoid making mistakes?
Choir Spokesperson: I would be happy to do that. I taught composition to high-schoolers, so
it will be fun! I can dust off my red pencil…
[The choir spokesperson reviewed the revised manuscript, which identified a few minor
mistakes, all of which were corrected in the final version]
[After reviewing the revised version of the paper, for which Medrxiv complained about some
remaining personal information]. Is there any other personal information that should be
removed?

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Yes, go ahead. All personal information that remains is OK to distribute.

Questions about the rehearsal space and layout
Question: The LA times mentions that the space is "about the size of a volleyball court"
(which is about 60 ft x 30 ft). And that "cushioned metal chairs extended in six rows of 20, with
about a foot between chairs and one aisle down the center. There were twice as many seats as
people." I've made the schematic below trying to match that description. Is this approximately
accurate? If not, can you point out the differences?
Choir Spokesperson: This is accurate. See attached pdf for rehearsal locations and infections.
[Not shown for privacy]
Question: What is the approximate height of the room?
Choir Spokesperson: Ceiling is vaulted, running the width of your diagram. High point
approx. between rows E and F, and is at about 20’. Ceiling height at room perimeter is about 10’
Question: Did the conductor face everyone as drawn? Did the conductor sign along?
Choir Spokesperson: The conductor faced everyone as shown. He does not sing along, but
sometimes sings a section to demonstrate correct notes/timing.
Question: Was the piano next to the conductor?
Choir Spokesperson: The piano is always located on his right. The accompanist sitting on
the bench is about 12’ from the conductor.
Question: I am pretty sure the answer is no, but one of the reviewers asked if there was video
surveillance in the hall, so I just wanted to confirm that there was not.
Choir Spokesperson: No
Question: Do you know where different people were seating that got sick and not? This
information would be extremely useful, even if not totally accurate. I added letters for the rows
and numbers for the columns of the chairs for ease of communication. Ideally you could print the
chart, and mark with an X the chairs where the occupant got sick, and a circle on the chairs that
had an occupant who did not get sick.

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Choir Spokesperson: See attached pdf with locations of people who got sick and not,
including the dates when known, and of the index case.
Question: If you know that people sitting on one part of the room got sick earlier or later, this
would be very useful.
Choir Spokesperson: Start dates indicated on diagram where known. Seems to be mostly on
3/13. Some people had such mild symptoms they did not know they were sick or had no
symptoms and tested positive.

Questions about the activities during the rehearsal
Question: In terms of the non-singing times, would the list below be approximately correct?
(And if not, please update)
- 6:20-6:30: Arrival of members in the ~10 min before the rehearsal, people chit-chatting
etc
- 6:30-7:10: First part of rehearsal, everyone together in one room
- 7:10: Breaking into bass/baritone and soprano/alto group, some talking along the way, but
not a break
- 7:10-8:00: Separate rehearsals for 50 min
- 8:00-8:15: 15 min break, people talking, having snacks
- 8:15-9:00: rehearsal with everyone together in one room
- 9:00: "Most attendees left the practice immediately after it concluded" per county report
Choir Spokesperson: This is accurate, but probably not exact. The break is usually only about
10 minutes. I have seen reports that say one group “stood around a piano” during the and that is
NOT correct. They sat in the church pews to sing and the piano was separated from the singers
by at least 10 feet.
Question: the LA times article mentions that "At one point the members broke into two
groups, each standing around separate pianos to sing." Was that on the same room? Were you any
closer to each other during that time? How long did that last?
Choir Spokesperson: This is incorrect. When we split into two groups, the tenors and basses
(mostly men) went with the conductor and our accompanist to the large church sanctuary, where
the conductor stood in the front facing the singers, who were seated in the pews. People did not
need to sit close together, since the room easily seats 150 and there were only around 20 singers.
The piano in that room is off to the side, with the accompanist seated about 20’ from where the
conductor was standing. The sopranos and altos all shifted to the right side of your diagram, so
were in rows A-F, seats 11-20. Again, there were only around 35 singers and 60 seats, so not all
seats were occupied. One of our singers led the sectional rehearsal from the piano. This lasted
about 45 minutes, and we took a brief break (10 min.) when we got back together. So we

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rehearsed in the same room for 45 minutes, split for 45 minutes, took a break for 10 minutes.
Then we were all in the same room rehearsing for 50 more minutes, until 9pm.
Question: one possible alternative to air transmission would be transmission through
touching objects. Even though you clearly tried to not touch objects of each other, it is still
important to learn a little more about this. The article mentions "Some members helped set up or
remove folding chairs. A few helped themselves to mandarins that had been put out on a table in
back." How many people help with the chairs?
Choir Spokesperson: Three people set up chairs. All 3 got sick on the 13th or 14th. Everyone
is responsible for putting their own chair away afterward. People fold their chair and carry it to a
rolling rack, where the chairs are hung on pegs. Sometimes, people will stand by the racks and
assist with placing the chairs on them. I am sure that happened that night. Then the racks are
wheeled into a storage area. Some people have others who take care of their chairs, so not
everyone who got sick touched a chair. (I have a bad shoulder, so my husband takes care of my
chair every week, along with others. I was sick on the 13th and he was sick on the 15th.)
The index case did not help with setting up or removing the chairs.
Question: How of the people touched the mandarins? Did they get sick or not?
Choir Spokesperson: About ½ of those in attendance had an orange. Both those who ate one
and those who did not got sick.
Question: Where there any other objects that many people could have touched? E.g. a door
handle for entering or exiting the room or building?
Choir Spokesperson: When we arrive, there is a singer who “greets” us and directs us to any
handouts or music we need to pick up. (There was a piece of music for the women that night.)
The exterior door is usually propped open during this time, since most of us arrive within about
10 minutes of each other. That means very few people needed to touch the door handle. When
we arrived on that evening, we were all directed to a large bottle of hand sanitizer, which most
everyone used.
Some people touched the interior door handles when going to and from the rehearsal room to
the sanctuary and to the restrooms, but since we moved in groups, a few held the door and most
did not have to touch it. Some people used the restrooms, so those doors were touched. Those
who did or did not use the restrooms all got sick.
Question: In terms of the periods with talking, how much mingling was there? Did people
talk to just a few friends, or did they talk to a lot of people?
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Choir Spokesperson: Since our break is so short, there is not a lot of time for mingling. If
people need to use the restrooms, that takes most of the time, so they might talk to the people in
line for the bathroom, but that’s it. Those who get a snack go to the table in the back, and then
leave the table to make room for others to access the goodies. I would say that, on average,
people speak to 2 or 3 people during the break. As I said above, the group is seriously about the
music, not overly social. Our director keeps strict control over the time we have to maximize the
learning.

Questions about activities of known index case
Question: We would like to include some additional info about the index case that should not
reveal the person’s identity: in particular, that the person did not have anyone sitting in the 3-6 ft
area immediately forward, where large droplets would have landed; that the person did not
handle the chairs or had snacks; and that the person did go to the bathroom (but that many who
did not go, also got ill). These help firm the case against the other routes of transmission.
Choir Spokesperson: The index case sat [at location X], so no one was in front of [them,
within the likely 9 feet landing radius of expired droplets] The person to [their] right was about 5
feet away, and the person to [their] left was about 4 feet away. [They] turned slightly to [one
side] to see the director and so [their] exhalations would have been more toward the person on
[one side], who [did contract COVID-19]. Our index case went to the bathroom. [They] used the
restroom off the hall, not the main restroom. [They did not help with the chairs that evening].
The restrooms off the hall [that the index case used], which are closer, are single men’s and
women’s rooms. The vast majority of singers go to the restrooms located a little further away
because they have multiple stalls. I am one of the people who uses the closer single restrooms. I
usually meet the same people coming and going during the break, and I would say there are only
about 6 people who use these regularly.
The index case did not have any snacks. [That evening that person had very limited
interactions with others]
Question: In particular, do we know how many people the index case may have talked to in
the periods with talking?
Choir Spokesperson: I talked with the person [they] told about [the index case’s] sore throat.
[The index case] arrived just in time to start. [They] sat [in a specific location], so I noticed
[them] coming in every week. [Their] talking to people was minimal [during the entire
rehearsal].
Question: I assume that the identity of the index case continues to be confidential, which is of
course appropriate. But I just wanted to ask in case it had somehow become public, which would
perhaps relax the restrictions on what we can say.
Choir Spokesperson: all the other choir members already know who it was, but there are
probably legal/privacy concerns we should heed!
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Questions about the rehearsal space
Question: It would be very useful to learn more about the heating / ventilation system in the
room. Does the room have forced air ventilation? Or radiators or other system? Was the system
in use during that rehearsal? Where doors or windows open during the rehearsal?
Choir Spokesperson: The room is heated by a relatively new commercial forced air furnace.
There is no other source of heat. The people who get there early to set up turned the heat up to 68
degrees. It was about 45 degrees outside, so the heat was on initially, but with so many people in
the room, it did not need to stay on to maintain the 68 degrees. During the rehearsal, from 6:30
to 9:00pm, no exterior doors were open.
Question: It would be very useful to talk to whoever maintains the heating and ventilation
system for the building, could you help us locate that contact information?
Choir Spokesperson: here is the contact info. [Removed for privacy]
Question: Was there anyone else in the building during the rehearsal, even if in different
rooms?
Choir Spokesperson: Those who set up for the rehearsal say the building was dark and locked
when they arrived at 6pm. We do not know of anyone else who was there during our rehearsal,
but someone could have been in a back office or room and we did not see them.

Questions related to the possibility of transmission outside the
choir rehearsal
Question: Was there anyone who may have had some very mild symptoms at the time of the
rehearsal? Or someone who got sick earlier than the rest? This would give us a candidate
location for the initial contagion. Understood that this is a very delicate subject. We don't want to
know anything about the person, but just the location within the room, the first / second piano
group, and whether they handled chairs mandarins / other objects or not would be useful.
Choir Spokesperson: Of all who responded, only one person said someone was coughing
behind [them], and that was during the women’s sectional. The one reporting this was seated in
X (removed for privacy) for that portion. Someone’s husband, who came to pick up his wife,
said there was someone coughing as they exited the building at 9pm. Almost everyone else said
they heard no coughing, sneezing, etc. during the entire time. If someone HAD coughed or
sneezed, most of us would have turned to look. We specifically told the singers not to attend if

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they had any symptoms whatsoever, even if they believed it to be just seasonal allergies. So we
were sensitive to that.
The index case was seated in X [removed for privacy] and told another singer that [they]
wasn’t feeling well and had a sore throat. During the sectional, [they] would have moved to Y
[removed for privacy]. The index case did not have an orange or move a chair, but [they] did use
the restroom located down the hall on the left side of your diagram.
Question: Did choir members see each other elsewhere during the week before the rehearsal?
Or did they carpool to the rehearsal? If so, how many may have done so? (The County Report
says that they investigated other possibilities, and concluded that the rehearsal was where
contagion happened, but they don't give any detail on this)
Choir Spokesperson: I think the county is correct in their assessment.
It is possible that people saw each other during the week, but our choir is comprised of
people from a large area who attend to sing. It is not a highly social group in that it is not
comprised of groups of friends who see each other all the time. It is pretty seriously about the
music. We don’t have social gatherings except for our summer picnic just before rehearsals start
in September. We have been in the group since 2005, and the only people we see socially are the
ones in our carpool! I know of several groups of us who drive from Anacortes, which is 14 miles
away from the rehearsal site. One carpool has 5, one has 4, one has 2. Other people live closer to
the church, so don’t need to pool. There may be a couple more groups, but I am not sure of that.
The county report mentions one person who attended the March 3rd but not the March 10th
rehearsal, and that fell ill with COVID-19. Did that person have some contact with the index
patient where transmission could have happened? Or any comments on that situation?
Choir Spokesperson: I am pretty sure no one had contact with the index case during that
week. I don’t know of anyone who got Covid before March 10 except our index case, but it is
apparent that the disease was in our area in early March.

Questions about the timing of the secondary cases
Question: There were 3 people who started having symptoms 1 day after the rehearsal,
according to the report from the public health agency. If you know who those are, where those
[from the group of singers that would have spent the most more time with the index case]. And
were they seated closer to the index case?
Choir Spokesperson: I don’t know of anyone who started having symptoms 1 day after
rehearsal. The earliest I heard about through our survey is the 12th, or two days afterward. That
would be a person right next to our index case in [location X] and [another person about 12 feet
away from the index case]. [Both were from same group of people where the index case was
during the separate 50 min. rehearsal]. The majority got sick on the 13th. I am not sure the
county health dept. is correct here.
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[To make sure] I called several people, and no one remembers hearing of someone who
was ill before Thursday, the 12th. Two people reported feeling ill on that day. Most felt
symptoms on Friday or Saturday. This is an excerpt from our director’s email to the group on
March 17; he is the person people were communicating with regarding the outbreak.
“Well, this is a little sooner than I'd expected to be in touch. We have some news of concern to
relate. At least 6 of us from the Chorale came down with a fever Friday night or Saturday,
including me. Two have been tested and the tests are being sent on to test for Coronavirus. We
are told we won't hear those results until Thursday; we will keep you apprised as to the results.”

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Information Provided by Dr. Lea Hamner of the Skagit County
Health Dept.
I added some more information below to present the complete picture of the small break-out
groups.
a. 2 people (one confirmed, one probable case) missing from the small room breakout totals
because we do not know which room they went to.
b. Large room: 42 people in it, including:
i. Index case
ii. 39/41 became ill after practice (of 39, 26 were confirmed and 13 were probable)
including:
a. 2 people who became ill on 3/11
b. 6 people who became ill on 3/12
c. Small room: 17 people in it, including:
i. 12/17 became ill after practice (of 12, 6 were confirmed and 6 were probable)
a. 1 person who became ill on 3/12

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References
● Ahmed K, Akhondzada A, Kurnitski J, Olesen B. Occupancy schedules for energy
simulation in new prEN16798-1 and ISO/FDIS 17772-1 standards. Sustainable Cities
and Society. 2017;35:134–144.
● ASHRAE. ANSI/ASHRAE Standard 62.1-2016 Ventilation for Acceptable Indoor Air
Quality. American Society of Heating, Refrigerating and Air Conditioning Engineers, Inc.
Atlanta, GA. 2016.
● Baughman AV, Gadgil AJ, Nazaroff WW. Mixing of a point source pollutant by natural
convection flow within a room. Indoor Air. 1994;4:114-122.
● AK Melikov, Human body micro-environment: The benefits of controlling airflow
interaction, Building and Environment. 2015;91:70-77.
● EPA. Exposure Factors Handbook, Table 6-2, Chapter 6. 2011.
https://www.epa.gov/expobox/exposure-factors-handbook-chapter-6

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