Pollán M, Pérez-Gómez B, Pastor-Barriuso R, et al. - Researchers conducted a nationwide population-based study investigating the seroprevalence of SARS-CoV-2 infection in Spain at national and regional level. Using two-stage random sampling, they selected 35,883 households from municipal rolls stratified by province and municipality size, with all residents invited to participate. From April 27 to May 11, 2020, a questionnaire on history of symptoms compatible with COVID-19 and risk factors was answered by 61,075 participants (75·1% of all contacted individuals within selected households); these participants received a point-of-care antibody test, and, if agreed, donated a blood sample for additional testing with a chemiluminescent microparticle immunoassay. Seroprevalence of 5·0% by the point-of-care test and 4·6% by immunoassay were observed, with a specificity–sensitivity range of 3·7% to 6·2%, with no disparities by gender; children younger than 10 years had lower seroprevalence (< 3·1% by the point-of-care test). Analyses suggest that the Spanish population, in majority, is seronegative to SARS-CoV-2 infection, even in hotspot areas. Detectable antibodies were noted in most PCR-confirmed cases, but a PCR test was not undertaken in a substantial proportion of people with symptoms compatible with COVID-19 and there were at least a third of infections detected by serology as asymptomatic. These findings highlights the necessity for maintaining public health measures to avoid a new epidemic wave.