Introduction
On Dec 31, 2019, a cluster of viral pneumonia cases, subsequently identified as coronavirus disease 2019 (COVID-19), was reported in Wuhan, China.1 Subsequent reports suggested that community transmission had occurred in Wuhan and Hubei province, leading to the lockdown of Wuhan on Jan 23, 2020, and subsequently other cities in Hubei province, to control COVID-19 spread.2, 3 WHO declared a public health emergency of international concern on Jan 30, 2020.4 As of March 6, 2020, 98 192 confirmed cases and 3380 deaths were reported globally, including 17 481 cases and 335 deaths from 88 regions or countries outside of mainland China.5
Singapore, a city-state in southeast Asia, developed case-definitions, laboratory tests, and enhanced surveillance to detect affected individuals with COVID-19 early, to enable containment of the disease. Singapore confirmed its first imported case of COVID-19 from Wuhan on Jan 23, 2020.6 With the lockdown of Wuhan, inbound flights from Wuhan to Singapore had ceased since Jan 23, 2020. As of Feb 22, 2020, 89 people in Singapore were confirmed to have COVID-19, and the first 18 affected individuals all reported recent travel to Wuhan. Singapore identified several local clusters of COVID-19, with the first three linked to a tour group from China, a company conference, and a church.
To contain the spread of COVID-19, several pertinent questions need to be addressed. What is the infectiousness of cases? What are the settings and activities beyond family clusters where spread can occur? What measures should be taken to improve surveillance and case-detection? What measures can reduce disease spread? To answer these questions, we report data for the first three clusters of COVID-19 cases in Singapore, the epidemiological and clinical investigations done to ascertain disease characteristics and exposure types, and summary statistics to characterise the incubation period of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and the serial interval between transmission pairs. Our findings will be important for countries and cities to calibrate detection and response efforts during the ongoing epidemic.