Transmission of SARS-CoV-2 by inhalation of respiratory aerosol in the Skagit Valley Chorale superspreading event

Abstract

During the 2020 COVID-19 pandemic, an outbreak occurred following attendance of a symptomatic index case at a regular 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 airborne route is likely. It is vital to identify features of cases such as this so as to better understand the factors that promote superspreading events. Based on a conditional assumption that transmission during this outbreak was by inhalation of respiratory aerosol, we use the available evidence to infer the emission rate of airborne infectious quanta from the primary source. We also explore how the risk of infection would vary with several influential factors: the rates of removal of respiratory aerosol by ventilation; deposition onto surfaces; and viral decay. The results indicate an emission rate of the order of a thousand quanta per hour (mean [interquartile range] for this event = 970 [680-1190] quanta per hour) and demonstrate that the risk of infection is modulated by ventilation conditions, occupant density, and duration of shared presence with an infectious individual.

Superspreading Events Without Superspreaders: Using High Attack Rate Events to Estimate Nº for Airborne Transmission of COVID-19

“…12 In the case of the Wuhan buses, the index patient is thought to have had a mild cough (Y. Yang, personal communication). 13 Note that in the Skagit choir case, the calculated quanta emission is of 2,347/h is roughly double the mean emission rate of 970 quanta/h reported by Miller et al [20] ; this difference is largely due to a mean assumed breathing rate in the Miller calculation of ~1.0 m 3 /h (range 0.65-1.38), compared to 0.5 m 3 /h here. based on the work of Buonnano et al , recommends a range of ~2-14 quanta/hour for oral breathing (depending on the activity) and 61-408 quanta/h for loud speaking [22,28] .…”

“…A similar effect has been suggested to occur if a low volume of virions are transmitted between masked individuals when social distancing is being practiced. [68] individuals to be higher than typical [20,21,29] . The inference of high viral loads in prior work stems from an assumption of low volumes of aerosols emitted by index patients (order 1-10 nL/hour) [20,21] .…”

“…[68] individuals to be higher than typical [20,21,29] . The inference of high viral loads in prior work stems from an assumption of low volumes of aerosols emitted by index patients (order 1-10 nL/hour) [20,21] . In contrast, based on Stadnytskyi's measurements, we estimate ~30 μL/hr emitted are emitted when speaking [33] .…”

“…While the absolute values of N 0 will change depending on the aerosolized volume assumption, this consistency would hold for any single value of the viral density chosen. [10,33] ; (b) range based on exhaled breath condensate (EBC) dilution measurements [34][35][36] ; Talking : (h) value used (2.7 × 10 -2 mL/hr); (i) range in Miller et al based on Morawska et al [10,20] ; (k) range in Stadnytskyi et al [33] ; Removal rate λ (1/hr) (k), (l) value used for cases in buildings and buses, respectively (2.12 h -1 and 3. Independent of the estimate of N 0 , and as Miller, Jimenez, Buannano, and Bazant have separately done, the case studies can be used to compute the number of aerosolized units of N 0 (quanta) released per unit time, which can in turn be used to estimate the quanta released by talking or breathing [20][21][22]28] .…”

“…(By "calibration quanta emission rates", we mean the quanta per hour emitted by the index patients in the case studies.) Miller et al calculate that, assuming that 1 quanta (1 N 0 ) is equal to 1000 viral RNA copies, the ~1000 quanta/hour estimated in the Skagit choir case would correspond to a viral load of ~10 11 copies/mL [20] . Buonanno similarly calculates that a quanta emission rate of 142 quanta/h, assuming N 0 = 50, corresponds to a viral load of 10 9 copies/mL (so that if N 0 = 1000, the viral load would be on the order of 10 10 -10 11 copies/mL [29] .)…”