Tracking SARS-CoV-2 seroprevalence in frontline healthcare workers at the beginning of COVID-19 pandemic and reliability of serological laboratory interpretation: a prospective cohort study

Abstract

Background: Frontline healthcare workers (HCWs) faced a high risk of SARS-CoV-2 infection during the early stages of the COVID-19 pandemic. This study aims to determine the prevalence and incidence of SARS-CoV-2 infection in HCWs using various serological methods and to evaluate diagnostic reliability. Methods: A prospective study was conducted on 240 frontline and 120 non-frontline HCWs at Siriraj Hospital in Bangkok, Thailand, from May 2020 to March 2021. Blood samples were collected at enrollment and at 4, 12 and 48 weeks. Three serological methods were utilized: electrochemiluminescence immunoassay (ECLIA) for qualitative anti-nucleocapsid protein antibodies, enzyme-linked immunosorbent assay (ELISA) and chemiluminescent microparticle immunoassay (CMIA) for quantitative anti-spike receptor-binding domain (RBD) antibodies. ELISA-positive samples were further analyzed with microneutralization and flow cytometry-based assays. Results: One non-frontline HCW had initial positive ECLIA result but tested negative by both ELISA and CMIA. Two frontline HCWs consistently tested positive by ELISA over these 48 weeks, resulting in a seroprevalence of 0.83% (2/240) among frontline HCWs. Ten frontline and six non-frontline HCWs had borderline or positive level of anti-RBD IgG by ELISA but were negative for microneutralization. A flow cytometry-based assay suggested that these positive ELISA results were likely due to cross-reactivity from pre-existing antibodies against other human coronaviruses (HCoVs), particularly HCoV-HKU1. Conclusions: An extremely low prevalence of SARS-CoV-2 infection among HCWs in Thailand at the pandemic’s outset, likely due to effective transmission control. Pre-existing antibodies against other human coronaviruses could lead to false positive serological testing for SARS-CoV-2.