What did we learn after the 2020 pandemic? Seroprevalence of SARS-CoV-2 infection in a North region of Portugal during 3rd lockdown

Background: Serological studies of antibody prevalence in response to infection with the acute respiratory syndrome coronavirus 2 (SARS-CoV-2) are useful monitor epidemic progression disease and evaluate rates. In this study, we estimated anti-SARS-CoV-2 antibodies (IgM IgG) a Portuguese sub-district, during National lockdown - January-March 2021.Methods: seroepidemiological survey participants were tested for SARS-CoV-2 IgG), blood samples. The seroprevalence results stratified by age, gender, education, occupational exposure, symptoms previous infection.Results: Seroprevalence was 41.3%, (15.7% IgG positive, 11.6% IgM positive 14% both). majority (74%) working presentially did serological assay because they had either / contact (90%) or antigen test (36%). From all seropositive cases only 44% symptomatic. Our show that SARS -CoV-2 is high North, parallel Surveillance System. higher woman than man adults older 21. Almost people at their regular places work reflecting danger exposure. Conclusions: These suggest that, between January March 2021, restriction conditions effective but unable stop epidemic. It essential assess population immunity if it lasts, specially from new COVID 19 variants.