In September 2020, we tested 13,398 persons in Portugal for antibodies against severe acute respiratory syndrome coronavirus 2 by using a quota sample stratified by age and population density. We found a seroprevalence of 2.2%, 3-4 times larger than the official number of cases at the end of the first wave of the pandemic.
Keywords: COVID-19; Portugal; SARS-CoV-2; antibodies; coronavirus disease; coronaviruses; pandemic; population prevalence; respiratory infections; seroepidemiology; severe acute respiratory syndrome coronavirus 2; viruses; zoonoses.