The aim of the population-based study was to estimate the seroprevalence -- the level of a pathogen in a population, as measured in blood serum -- of SARS-CoV-2 infection in Spain at national and regional level.
About 35,883 households were selected from municipal rolls using two-stage random sampling stratified by province and municipality size, the researchers said.
From April 27 to May 11,61,075 participants answered a questionnaire on history of symptoms compatible with Covid-19 and risk factors, among others.
"Prevalences of IgG antibodies were adjusted using sampling weights and post-stratification to allow for differences in non-response rates based on age group, sex, and census-tract income," the researchers wrote in the medical journal.
"Using results for both tests, we calculated a seroprevalence range maximising either specificity (positive for both tests) or sensitivity (positive for either test)," they said.
The researchers noted that seroprevalence was 5 per cent by the point-of-care test and 46 per cent by immunoassay, with no differences by sex and lower seroprevalence in children younger than 10 years.
There was substantial geographical variability, with higher prevalence around Madrid (over 10 per cent) and lower in coastal areas (less than 3 per cent), they said.
The findings suggest that majority of the Spanish population is seronegative to SARS-CoV-2 infection, even in hotspot areas, according to the researchers.
"Most PCR-confirmed cases have detectable antibodies, but a substantial proportion of people with symptoms compatible with Covid-19 did not have a PCR test and at least a third of infections determined by serology were asymptomatic," the researchers wrote in the journal.
"These results emphasise the need for maintaining public health measures to avoid a new epidemic wave," they said.