Investigation of Three COVID-19 Infection Clusters in Singapore

Knowledge about infectiousness of COVID-19 cases and typical transmission settings and scenarios is still limited. Consequently, uncertainty remains about which measures should be taken to reduce disease spread. To address these questions, researchers analyzed the first three clusters of COVID-19 cases in Singapore in January and early February 2020. Cases were identified by testing symptomatic patients in hospitals and in sentinel primary-care clinics. Researchers traced the cases' contacts from 14 days before symptom onset until isolation.

Cluster A (11 cases) was related to Chinese tourists visiting a shop, cluster B (20 cases) was linked to an international business meeting, and Cluster C (5 cases) occurred in a church with 2 visitors from Wuhan, China. The 425 close contacts of cases in the clusters were quarantined and tested for SARS-CoV-2. Of these contacts, only two, associated with cluster B, tested positive. Analysis of disease transmission revealed a median incubation period of 4 days (interquartile range, 3–6 days). Three transmission pairs from a cluster case to a household member had serial intervals between 3 and 8 days. A single case transmitted the virus to 11 individuals (31% of all transmissions), and one case each caused 8, 7, 4, 3, and 2 transmissions. Direct physical contact was identified in many transmission pairs (e.g., between shop assistant and customer or during team-building activities, including handshaking or sharing of meals), but using the same church seat was also identified as a possible mode of transmission. Of the case patients, 19 subsequently travelled to China, Malaysia, France, Spain, and the U.K.