On April 1, 2020, this report was posted online as an MMWR Early Release.Wycliffe E. Wei, MPH1,2; Zongbin Li, MBBS1; Calvin J.
Chiew, MPH1; Sarah E. Yong, MMed1; Matthias P. Toh, MMed2,3; Vernon J. Lee, PhD1,3 (View author affiliations)Preliminary evidence indicates the occurrence of presymptomatic transmission of SARS-CoV-2, based on reports of individual cases in China.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.The possibility of presymptomatic transmission increases the challenges of containment measures.