Superspreading of SARS-CoV-2 at a choir rehearsal in Finland-A computational fluid dynamics view on aerosol transmission and patient interviews

Abstract

Introduction: COVID-19 pandemic has highlighted the role of aerosol transmission and the importance of superspreading events. We analyzed a choir rehearsal in November 2020, where all participants, except one who had recently earlier recovered from COVID-19, were infected. We explore the risk factors for severe disease in this event and model the aerosol dispersion in the rehearsal room.

Materials and methods: Characteristics of participants were collected by interviews and supplemented with patient records. A computational simulation of aerosol distribution in the rehearsal room and the efficacy of potential safety measures was conducted using the Large-Eddy Simulation approach. Infection risk was studied by analyzing quanta emission and exposure with the Wells-Riley equation.

Results: The simulation showed that airborne transmission likely explains this mass contagion event. Every singer was exposed to the virus in only 5 min from the beginning of the rehearsal, and maximum concentration levels were reached at 20 min the concentration levels started to approach a steady state after 20 min. Although concentration differences existed in the room, risk levels near (1 m) and far (5 m) from the aerosol source were similar for certain singers. Modeling indicated infection risk levels of 70-100% after one hour; the risk would have been considerably reduced by wearing high-filtration respirators. Age and pre-existing comorbidities predicted more severe disease. The high incidence of illness may be partly attributed to the relatively high median age of individuals. Additionally, those admitted to the hospital had multiple underlying health conditions that predispose them to more severe disease.

Conclusions: Airborne transmission and indoor space can explain this mass exposure event. High-filtration respirators could have prevented some infections. The importance of safety distances diminishes the longer the indoor event. The concept of safety distance is challenging, as our study suggests that long range airborne transmission may occur in indoor events with extended duration. We encourage informing the public, especially persons at risk, of safety measures during epidemics.

Fig 1

(a) Room geometry with 11 individuals showing the aerosol cloud dispersion at time t = 20 min. The dimensions of the room are 12m x 12m x 3m and the individuals are spaced 2–3 m apart from each other. (b) A cut plane of the room and aerosol concentration level are expressed as quanta ([q] = 1/m³). The scale is truncated at q = 25 1/m³. i: index person, ii: inflow air jets, iii: exhaust air, iv: radiators.

Fig 2. Average aerosol concentration field (q) on breathing level from time interval t = 20–25 min when the aerosol concentration levels have reached a fully developed, saturated state.

The room concentration levels are directly linked to the mask filtration efficiency. The upper bound of the scale is truncated at the value q = 20 1/m³.

Fig 3

Infection risk as a function of time in four different scenarios: (a) no masks, (b) cloth masks, (c) surgical masks, and (d) FFP2/N95 masks. As shown in the inset of panel (a), singer 6 stands in front of the infected person (singer 3), while singer 2 stands next to singer 3. The colouring of the line plots is based on the distance of the singers to the index person, as shown in panel (d). A theoretical average concentration is shown as the black dashed line.