Prevalence of SARS-CoV-2 antibodies among workers of the public higher education institutions of Porto, Portugal

Meireles, P.; Amaro, J.; Costa, J. P.; Mendes Lopes, M.; Varandas, T.; Norton, P.; Guimaraes, J. T.; Severo, M.; Barros, H.

Objectives: To assess the prevalence of SARS-CoV-2 specific immunoglobulin (Ig) M and IgG antibodies among workers of the three public higher education institutions of Porto, Portugal, from May to July 2020. Methods: A rapid point of care testing for specific IgM and IgG antibodies of SARS-CoV-2 was performed, and a questionnaire was applied to 4592 workers on a voluntary basis. We computed the apparent IgM, IgG, and combined IgM or IgG prevalence, along with the true prevalence and 95% credible intervals (95% CI) using Bayesian inference . Results: We found an apparent prevalence of 3.1% for IgM, 1.0% for IgG, and 3.9% for either antibody class. The estimated true prevalence was 2.0% (95% CI 0.1-4.3) for IgM, 0.6% (95% CI 0.0-1.3) for IgG and 2.5% (95% CI 0.1-5.3) for IgM or IgG. A SARS-CoV-2 molecular diagnosis was reported by 21 (0.5%) workers, and of these, 90.5% had a reactive IgG result. Seroprevalence was higher among those reporting known contacts with confirmed cases, having been quarantined, having a previous molecular negative test, or having had symptoms. Conclusions: We found a low seroprevalence among workers from the three public higher education institutions of Porto after the first wave of the SARS-CoV-2 infection. However, the estimated true seroprevalence was approximately five times higher than the reported SARS-CoV-2 infection using a molecular test.