• Sarah Ee Fang Yong, Danielle Elizabeth Anderson, Wycliffe E Wei, Junxiong Pang, Wan Ni Chia, Chee Wah Tan, Yee Leong Teoh, Priyanka Rajendram, Matthias Paul Han Sim Toh, Cuiqin Poh, Valerie T J Koh, Joshua Lum, Nur-Afidah Md Suhaimi, Po Ying Chia, Mark I-Cheng Chen, Shawn Vasoo, Benjamin Ong, Yee Sin Leo, Linfa Wang, and Vernon J M Lee.
• Ministry of Health, Singapore; National University Health System, Singapore.
• Lancet Infect Dis. 2020 Jul 1; 20 (7): 809-815.

BackgroundElucidation of the chain of disease transmission and identification of the source of coronavirus disease 2019 (COVID-19) infections are crucial for effective disease containment. We describe an epidemiological investigation that, with use of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) serological assays, established links between three clusters of COVID-19.MethodsIn Singapore, active case-finding and contact tracing were undertaken for all COVID-19 cases. Diagnosis for acute disease was confirmed with RT-PCR testing. When epidemiological information suggested that people might have been nodes of disease transmission but had recovered from illness, SARS-CoV-2 IgG serology testing was used to establish past infection.FindingsThree clusters of COVID-19, comprising 28 locally transmitted cases, were identified in Singapore; these clusters were from two churches (Church A and Church B) and a family gathering. The clusters in Church A and Church B were linked by an individual from Church A (A2), who transmitted SARS-CoV-2 infection to the primary case from Church B (F1) at a family gathering they both attended on Jan 25, 2020. All cases were confirmed by RT-PCR testing because they had active disease, except for A2, who at the time of testing had recovered from their illness and tested negative. This individual was eventually diagnosed with past infection by serological testing. ELISA assays showed an optical density of more than 1·4 for SARS-CoV-2 nucleoprotein and receptor binding domain antigens in titres up to 1/400, and viral neutralisation was noted in titres up to 1/320.InterpretationDevelopment and application of a serological assay has helped to establish connections between COVID-19 clusters in Singapore. Serological testing can have a crucial role in identifying convalescent cases or people with milder disease who might have been missed by other surveillance methods.FundingNational Research Foundation (Singapore), National Natural Science Foundation (China), and National Medical Research Council (Singapore).Copyright © 2020 Elsevier Ltd. All rights reserved.

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