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Singapore: Seven Clusters of COVID-19 Cases Suggesting Presymptomatic Transmission

The possibility of presymptomatic transmission of SARS-CoV-2 increases the challenges of COVID-19 containment measures, which are predicated on early detection and isolation of symptomatic persons. The magnitude of this impact is dependent upon the extent and duration of transmissibility while a patient is presymptomatic, which, to date, have not been clearly established. 

This investigation identified seven clusters of COVID-19 in Singapore in which presymptomatic transmission likely occurred. Among the 243 cases of COVID-19 reported in Singapore as of March 16, 157 were locally acquired; 10 of the 157 (6.4%) locally acquired cases are included in these clusters and were attributed to presymptomatic transmission.

Presymptomatic transmission might occur through generation of respiratory droplets or possibly through indirect transmission. Speech and other vocal activities such as singing have been shown to generate air particles, with the rate of emission corresponding to voice loudness. News outlets have reported that during a choir practice in Washington on March 10, presymptomatic transmission likely played a role in SARS-CoV-2 transmission to approximately 40 of 60 choir members.

Environmental contamination with SARS-CoV-2 has been documented, and the possibility of indirect transmission through fomites by presymptomatic persons is also a concern. Objects might be contaminated directly by droplets or through contact with an infected person’s contaminated hands and transmitted through nonrigorous hygiene practices.

Containment measures should account for the possibility of presymptomatic transmission by including the period before symptom onset when conducting contact tracing. These findings also suggest that to control the pandemic it might not be enough for only persons with symptoms to limit their contact with others because persons without symptoms might transmit infection. Finally, these findings underscore the importance of social distancing in the public health response to the COVID-19 pandemic, including the avoidance of congregate settings.

Presymptomatic Transmission of SARS-CoV-2 — Singapore, January 23–March 16, 2020. Morbidity and Mortality Weekly Report (MMWR). Published 1 April 2020.

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