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A Population-Based Seroepidemiological Study of SARS-CoV-2 in Spain (ENE-COVID)
20 Pages Posted: 14 Jul 2020More...
Background: Spain is one of the European countries most affected by COVID-19 pandemic. Serological surveys are a valuable tool to assess the extent of the epidemic, given the existence of asymptomatic cases and the limited access to diagnostic tests. This nation-wide population-based study aims to estimate the seroprevalence of SARS-CoV-2 infection in Spain at national and regional level (province).
Methods: Residents in 35,893 households were selected from municipal rolls using two-stage random sampling stratified by province and municipality size. Participants (74·9% of contacted individuals) answered a questionnaire, received a Point-of-Care (POC) antibody test and, if agreed, donated a blood sample for additional testing with a chemiluminescent microparticle immunoassay. Prevalences (IgG antibodies) were adjusted using sampling weights and post-stratification to allow for differences in nonresponse rates based on age, sex, and census-tract income. Using results for both tests, we propose a prevalence range maximizing either specificity (positivity for both tests) or sensitivity (positive for either test) (Sp-Sn-range).
Findings: Seroprevalence among the 60,979 participants was 5% (POC) and 4.6% (immunoassay) (Sp-Sn-range 3·7% to 6·2%), with no differences by sex or age except lower figures in children. There was substantial geographical variability, with higher prevalences around Madrid (>10%) and lower in coastal areas (<2%). Seroprevalence among participants with a past positive PCR was 89% (Sp-Sn-range 87% to 91%), and, in those with ≥3 symptoms 17% (Sp-Sn-range 15% to 20%). Around 33% of participants with a positive result were asymptomatic (Sp-Sn-range 21% to 36%).
Interpretation: The majority of the Spanish population is seronegative to SARS-CoV-2 infection, even in hot-spot areas. Most PCR-confirmed cases have detectable antibodies, but a substantial proportion of people with symptoms compatible with COVID19 did not have a PCR and one third of infections determined by serology were asymptomatic. These results emphasize the need of maintaining public health measures to avoid a new epidemic wave.
Funding Statement: Spanish Ministry of Health, Institute of Health Carlos III & Spanish National Health System.
Declaration of Interests: The authors declare no competing interests.
Ethics Approval Statement: The Institutional Review Board of the Institute of Health Carlos III approved the study (Register number: PI 39_2020). The Spanish Agency for Personal Protection was consulted. Written informed consent was obtained from all study participants.
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