What is already known about this topic?: Preliminary evidence indicates the occurrence of presymptomatic Transmission of SARS-CoV-2, based on reports of individual cases in China.

What is added by this report?: Investigation of all 243 cases of COVID-19 reported in Singapore during January 23–March 16 identified seven clusters of cases in which presymptomatic Transmission is the most likely explanation for the occurrence of secondary cases.

What are the implications for public health practice?: The possibility of presymptomatic Transmission increases the challenges of containment measures. Public health officials conducting contact tracing should strongly consider including a period before symptom onset to account for the possibility of presymptomatic Transmission. The potential for presymptomatic Transmission underscores the importance of social distancing, including the avoidance of congregate settings, to reduce COVID-19 spread.

Presymptomatic Transmission of SARS-CoV-2, the Virus that causes coronaVirus disease 2019 (COVID-19), might pose challenges for disease control. The first case of COVID-19 in Singapore was detected on January 23, 2020, and by March 16, a total of 243 cases had been confirmed, including 157 locally acquired cases. Clinical and epidemiologic findings of all COVID-19 cases in Singapore through March 16 were reviewed to determine whether presymptomatic Transmission might have occurred. Presymptomatic Transmission was defined as the Transmission of SARS-CoV-2 from an infected person (source patient) to a secondary patient before the source patient developed symptoms, as ascertained by exposure and symptom onset dates, with no evidence that the secondary patient had been exposed to anyone else with COVID-19. Seven COVID-19 epidemiologic clusters in which presymptomatic Transmission likely occurred were identified, and 10 such cases within these clusters accounted for 6.4% of the 157 locally acquired cases. In the four clusters for which the date of exposure could be determined, presymptomatic Transmission occurred 1–3 days before symptom onset in the presymptomatic source patient. To account for the possibility of presymptomatic Transmission, officials developing contact tracing protocols should strongly consider including a period before symptom onset. Evidence of presymptomatic Transmission of SARS-CoV-2 underscores the critical role social distancing, including avoidance of congregate settings, plays in controlling the COVID-19 pandemic.

Suggested citation for this article: Wei WE, Li Z, Chiew CJ, Yong SE, Toh MP, Lee VJ. Presymptomatic Transmission of SARS-CoV-2 — Singapore, January 23–March 16, 2020. MMWR Morb Mortal Wkly Rep. ePub: 1 April 2020. DOI: http://dx.doi.org/10.15585/mmwr.mm6914e1

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