SARS-CoV-2 seroprevalence and antibody kinetics among health care workers in a Spanish hospital after three months of follow-up.

BACKGROUND: At the COVID-19 pandemic peak in Spain, prevalence of SARS-CoV-2 infection in a cohort of 578 randomly selected health care workers (HCW) from Hospital Clinic de Barcelona was 11.2%. METHODS: A follow-up survey one month later (April-May 2020) measured infection by rRT-PCR and IgM, IgA, IgG to the receptor-binding domain of the spike protein by Luminex. Antibody kinetics, including IgG subclasses, was assessed till month 3. RESULTS: At month 1, the prevalence of infection measured by rRT-PCR and serology was 14.9% (84/565) and the seroprevalence 14.5% (82/565). We found 25 (5%) new infections in participants without previous evidence of infection (501). IgM, IgG and IgA levels declined in 3 months (antibody decay rates 0.15 (95% CI, 0.11; 0.19), 0.66 (95% CI, 0.54; 0.82), 0.12 (95% CI, 0.09; 0.16), respectively), and 68.33% of HCW had seroreverted for IgM, 3.08% for IgG, and 24.29% for IgA. The most frequent subclass responses were IgG1 (highest levels) and IgG2, followed by IgG3, and only IgA1 but no IgA2 was detected. CONCLUSIONS: Continuous and improved surveillance of SARS-CoV-2 infections in HCW remains critical, particularly in high-risk groups. The observed fast decay of IgA and IgM levels have implications for seroprevalence studies using these isotypes.