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

TLDR

During the 2020 COVID‑19 pandemic, a symptomatic index case at a Skagit Valley Chorale rehearsal on 10 March sparked an outbreak among 61 attendees. The study aims to identify characteristics that promote superspreading events by examining this outbreak. The authors assume aerosol inhalation from a single index case dominated transmission, estimate the emission rate of infectious quanta, and evaluate how ventilation, event duration, and surface deposition influence infection risk. The outbreak infected 53 of 61 attendees (including two deaths), the estimated emission rate was 970 ± 390 quanta/h, and increasing the aerosol loss rate to 5 h⁻¹ while shortening the event to 1 h could reduce infection risk by half.

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. 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.