Persistence of IgG response to SARS-CoV-2

Participants are tested on a monthly basis for the presence of SARS-CoV-2 with quantitative RT-PCR (RT-qPCR) and for antibodies targeting S1 (spike subunit 1) protein with a commercial semi-quantitative ELISA (Euroimmun IgG; Medizinische Labordiagnostika, Lübeck, Germany), using a stringent manufacturer-defined cut-off for having a positive test result (ratio ≥1·1; NCT04373889).6 By the end of September, 2020, seven rounds of testing had been done. To assess the longevity of the humoral immune response, we recorded the duration of the presence of detectable IgG in the serum of health-care workers who were seropositive for SARS-CoV-2. Of note, of the 13 individuals with no detectable neutralising antibodies, eight had weak neutralising antibody titres (NT50 55–100) and five had no measurable neutralising antibody titres from the start. Since antibodies specific for SARS-CoV-2 were only assessed for S1 protein, and because S1-specific cross-reactivity of prepandemic serum samples from patients infected with common cold human coronaviruses has been described,7,8 an explanation could be that these five individuals are false-positive for SARS-CoV-2 antibodies.