Prevalence and risk factors of SARS-CoV-2 antibody responses and asymptomatic carriers among a cohort of 2,455 healthcare workers of a general hospital during the Covid-19 Pandemic

Observational and prospective study to establish an immunological map of SARS-CoV-2 exposure of 2,455 healthcare workers (HCW) at the Maresme Health Consortium (CSdM), Catalonia, during COVID-19 pandemic, with serum sampling for antibody determination. In pre-vaccination serum samples, we performed an initial chemiluminescence immunoassay (CLIA) to detect: IgG+IgM+IgA and, in those with positive results, a differential analysis of IgM+IgA vs IgG by ELISA to detect an acute or past infection. A PCR was performed in nasopharyngeal swabs in positive IgM+IgA to confirm the non-diagnosed carriers of the virus. In post-vaccination samples, we assessed IgG SARS-CoV2 SPIKE-TRIMERIC and total antibodies to the nucleocapsid (N) protein of SARS-CoV-2. The individual occupational, social and health risk factors were assessed by a self-administrated epidemiological survey at each serum sampling period. Relatively large sample size, high participation rate in all phases of the study with four blood samples over time, high vaccination acceptance, and follow-up of the study cohort for 17 months (6-7 months before and 10-11 months after vaccination), represent important strengths of the study. As this is a study with an open cohort (with entrances and exits), it has made it possible to obtain an accurate picture of the epidemiological characteristics of the pandemic among CSdM HCW in real life. However, this open cohort design has made it difficult to assess risk factors that, on the other hand, have shown some changes over time due to progressive implementation of protective and preventive measures. Moreover, this is not a clinical trial but an observational study, so it is not the best design to assess the efficacy of a vaccine. However, it allows us to assess its effectiveness in terms of immunological response in real conditions.