Study design
Since Jan 2, 2020, the Ministry of Health in Singapore has been refining local case-definitions for people suspected of having COVID-19 (appendix p 4). Doctors are required by law to notify the Ministry of Health of cases of COVID-19.7 Further, Singapore implemented enhanced surveillance on Jan 31, 2020, to test for COVID-19 among patients in intensive care units and people who died of unknown cause, all people with pneumonia in hospitals, and individuals with influenza-like illness in sentinel primary-care clinics. Hospital-based doctors also have permission to test patients who they view with suspicion for clinical or epidemiological reasons. A confirmed case of COVID-19 is defined as an individual with a respiratory sample positive for SARS-CoV-2, using a laboratory-based PCR test (appendix p 1). Phylogenetic analysis is done of available viral genomic sequences to ascertain genetic clustering.
Research in context
Evidence before this study
We searched PubMed up to Feb 15, 2020 for reports published in English with no date limitations using the keywords “COVID-19” and “cluster”. A family cluster of coronavirus disease 2019 (COVID-19) was reported in Shenzhen, Guangdong. Transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has also been reported among co-workers in Germany.
Added value of this study
We analysed three clusters of COVID-19 in Singapore to assess interactions and possible modes of transmission. Our findings show that SARS-CoV-2 is transmissible in community settings beyond household clusters. Importation and limited local transmission of SARS-CoV-2 could have occurred before the lockdown of Wuhan and China's institution of travel restrictions.
Implications of all the available evidence
Countries should focus on enhancing detection and containment of local clusters of COVID-19, through surveillance of individuals with general pneumonia or influenza-like illnesses, and of people who have been in contact with unwell travellers from China. It is also important for countries to do active case-finding among close contacts of affected individuals, including contacts with mild symptoms, to contain clusters and stop them from spreading.
The Ministry of Health in Singapore identified the first three clusters of COVID-19 cases on Feb 3, 2020, Feb 5, 2020, and Feb 8, 2020. All epidemiological investigations and outbreak containment measures were implemented under the Infectious Diseases Act,7 which provides for use of data for analysis to control outbreaks.