Seven epidemiologic clusters of coronavirus disease 2019 (COVID-19) that were likely caused by presymptomatic transmission have been identified in Singapore. Within these clusters 10 such cases accounted for 6.4% of the 157 locally acquired cases identified in the country by March 16, 2020. This according to data published the Morbidity and Mortality Weekly Report as an MMWR Early Release.

Presymptomatic transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causative agent of COVID-19 may pose significant challenges for control of the pandemic. Singapore detected its first case of COVID-19 on January 23, 2020 and by March 16 a total of 243 cases were confirmed including the 157 locally acquired cases. Investigators reviewed the clinical and epidemiologic findings in all of these cases to determine whether presymptomatic transmission might have occurred.

Researchers defined presymptomatic transmission as the transmission SARS-CoV-2 from an infected 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 any other individual with the infection.


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Seven clusters with likely presymptomatic transmission were identified, within which 18 such cases were found. Presymptomatic transmission occurred 1 to 3 days before symptom onset in 4 clusters for which the date of exposure could be determined. Two clusters occurred between individuals who met or were bench-sharers at church; 2 clusters occurred as a result of exposure during a singing class; 3 clusters were associated between spouses/housemates; 1 cluster was the result of an encounter between one presymptomatic man and a woman.

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The possibility of presymptomatic transmission evidenced by the results from Singapore and other studies increases the challenges associated with containing this disease. According to the investigators, “public health officials conducting contact tracing should strongly consider including a period before symptom onset to account for the possibility of presymptomatic transmission.” These findings also underscored the importance of social distancing measures and the avoidance of congregate settings for halting the spread of COVID-19.

Reference

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. 2020; 69: 411-415.