ENE-COVID nationwide serosurvey served to characterize asymptomatic infections and to develop a symptom-based risk score to predict COVID-19

•In Spain, nearly 30% of SARS-CoV-2 infections in the first wave were asymptomatic.•Asymptomatic infections were more frequent in areas with lower viral circulation.•Men, young and old people, and smokers had more asymptomatic infections.•We developed a symptom-based score applicable in primary care or community settings.•A score ≥3 detects over 70% of cases among symptomatic people with specificity >70%. To characterize asymptomatic SARS-CoV-2 infections and develop a symptom-based risk score useful in primary healthcare. Sixty-one thousand ninty-two community-dwelling participants in a nationwide population-based serosurvey completed a questionnaire on COVID-19 symptoms and received an immunoassay for SARS-CoV-2 IgG antibodies between April 27 and June 22, 2020. Standardized prevalence ratios for asymptomatic infection were estimated across participant characteristics. We constructed a symptom-based risk score and evaluated its ability to predict SARS-CoV-2 infection. Of all, 28.7% of infections were asymptomatic (95% CI 26.1–31.4%). Standardized asymptomatic prevalence ratios were 1.19 (1.02–1.40) for men vs. women, 1.82 (1.33–2.50) and 1.45 (0.96–2.18) for individuals