Laboratory testing
Blood was collected from all participants at enrolment, and serum was stored at -20°C until testing. Sera samples were tested for anti-spike antibodies with Wantai SARS-CoV-2 total antibody ELISA (Beijing Wantai Biological Pharmacy, Beijing, China) and for anti-nucleocapsid antibodies using Platelia SARS-CoV-2 total antibody ELISA (Bio-Rad Laboratories, Hercules, CA) in the InterDiagnostic Clinic Laboratory, Baku. The manufacturer’s recommended controls and thresholds were used to define seropositivity. For participants who had haemolysed or insufficient specimens, repeat blood draws were performed within 30 days of enrolment.
A subset of SARS-CoV-2 PCR-positive study samples was sent to the Charite University Institute of Virology laboratory in Berlin, Germany, for whole genome sequencing (WGS).