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Increasing evidence points to transmission of SARS-CoV-2, the coronavirus that causes COVID-19, by asymptomatic or presymptomatic persons. Two studies contribute to the concept that these persons can transmit infection by producing respiratory droplets or aerosols with what would be considered minimal, nontussive respiratory effort.
Hamner and colleagues report the epidemiology of one of the first outbreaks of COVID-19 in the U.S. This outbreak occurred in Skagit County, Washington, in March 2020 and was eventually traced back to a choir practice involving 61 persons, 1 of whom had COVID-19 symptoms during the 2.5-hour encounter. In all, investigators identified 53 affected attendees, including 33 confirmed and 20 probable cases, representing a secondary attack rate of 53.3% for confirmed cases and 86.7% overall. The encounter involved many socializing events and opportunities, including close seating arrangements, and no precautions were taken at this very early event in the history of the epidemic in the U.S. Two choir practices within a 2-week period were thought to have amplified the number of infected choir members. The authors calculated that the second practice was most likely a superspreader event.
Stadnytskyi and colleagues used laser light scattering to attempt to measure the amount and size of potentially infectious particles generated by loud speech. Normal speaking produces thousands of fluid droplets with broad size distribution. The rate at which these droplets fall is directly related to their size. This size may decrease rapidly as the droplets dehydrate due to evaporation, yielding a longer lifetime (suspension time) in air. Using droplet size and number data, and calculations of the chance that a droplet of a specific size will contain at least one SARS-CoV-2 infectious particle, the authors estimate that 1 minute of loud speech generates at least 1000 virion-containing droplet nuclei that will remain airborne for at least 8 minutes. Smaller-than-average droplets may remain airborne for much longer, and patients with higher airway viral load may produce droplets with a higher viral concentration than were used in these calculations.
Hamner L et al. High SARS-CoV-2 attack rate following exposure at a choir practice— Skagit County, Washington, March 2020. MMWR Morb Mortal Wkly Rep 2020 May 15; 69:606. (https://doi.org/10.15585/mmwr.mm6919e6)
Stadnytskyi V et al. The airborne lifetime of small speech droplets and their potential importance in SARS-CoV-2 transmission. Proc Natl Acad Sci U S A 2020 May 13; [e-pub]. (https://doi.org/10.1073/pnas.2006874117)
Comment
Although these studies had vastly different approaches, they both bear out the concept that SARS-CoV-2 is readily transmissible through speech and such praiseworthy practices as choral singing. These findings, in turn, make the practices of social distancing and mask wearing essential, especially as we “open up society.”