This study was a nationwide, population based study in Spain that utilized  two-stage random sampling stratified by province and municipality size. From April 27 to May 11, 2020, 61,075 participants (75·1% of all contacted individuals within selected households) answered a questionnaire. 



Seroprevalence was 5·0% (95% CI 4·7–5·4) by the point-of-care test and 4·6% (4·3–5·0) by immunoassay. There was substantial geographical variability, with higher prevalence around Madrid (>10%) and lower in coastal areas (<3%). 



In 7273 individuals with anosmia or at least three symptoms, seroprevalence ranged from 15·3% (13·8–16·8) to 19·3% (17·7–21·0). Around a third of seropositive participants were asymptomatic, ranging from 21·9% (19·1–24·9) to 35·8% (33·1–38·5). Only 19·5% (16·3–23·2) of symptomatic participants who were seropositive by both the point-of-care test and immunoassay reported a previous PCR test.

Summary for Prevalence of SARS-CoV-2 in Spain (ENE-COVID): a nationwide, population-based seroepidemiological study

University of Michigan - Ann Arbor

The seroprevalence of COVID-19 is the measurement of the level of the pathogen in a population. There have been difficulties with estimating the seroprevalence in large populations because of the unknown qualities of COVID-19, expense in widespread population collection, asymptomatic infection, etc.

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