Contrasting SARS-CoV-2 epidemics in Singapore: Cohort studies in migrant workers and the general population

Background: From January 2020, Singapore implemented comprehensive measures to suppress SARS-CoV-2. Community transmission has been limited, although explosive outbreaks have occurred in migrant worker dormitories. Methods: We conducted longitudinal SARS-CoV-2 serology studies among 478 residents of a SARS-CoV-2 affected migrant worker dormitory between May and July 2020, and 937 community-dwelling adult Singapore residents with sera collected before September 2019 and in November/December 2020. We tested sera for SARS-CoV-2 neutralising antibodies and used Bayesian methods to estimate infection incidence accounting for test sensitivity and specificity. Findings: We estimated that <2 per 1000 adult residents in the community were infected with SARS-CoV-2 in 2020 (cumulative seroprevalence: 0.16% (95% CrI: 0.008% - 0.72%), approximately 4 times higher than the national notified case incidence. In contrast, in the migrant worker cohort, nearly two-thirds had been infected by July 2020 (cumulative seroprevalence: 63.8% (95% CrI: 57.9% - 70.3%); no symptoms were reported in almost all of these infections. Conclusions: Our findings demonstrate that SARS-CoV-2 suppression is possible with rapid implementation of border restrictions, case isolation, contact tracing, quarantining and social distancing measures. However, the risk of large-scale epidemics in densely-populated environments requires specific consideration in preparedness planning. Prioritising these settings in vaccination strategies should minimise risk of future resurgences and potential for spillover to the wider community. Funding: This work was funded by the Wellcome Trust grant (221013/Z/20/Z), the NUS Efforts Against COVID-19 fund and the Singapore National Medical Research Council (STPRG-FY19-001 and COVID19RF-003). Declaration of Interests: L-FW and WNC are co-inventors of a patent application for the cPass test kit. The remaining authors declare no competing interests in relation to this work. Ethics Approval Statement: The community cohort study was approved by the National University of Singapore Institutional Review Board (reference H-20-032).