Perez-Gomez, BeatrizPastor-Barriuso, RobertoPerez-Olmeda, MayteHernán, Miguel AOteo-Iglesias, JesusFernandez de Larrea-Baz, NereaFernandez-Garcia, AuroraMartín, MarianoFernandez-Navarro, Pablo LCruz, IsraelSanmartín, Jose LPaniagua-Caparros, Jose LeonMuñoz-Montalvo, Juan FBlanco, FaustinoYotti-Alvarez, RaquelPollan-Santamaria, MarinaENE-COVID Study Group2022-05-092022-05-092021-11J Clin Epidemiol. 2021 Nov;139:240-254.http://hdl.handle.net/20.500.12105/14323Objectives: To characterize asymptomatic SARS-CoV-2 infections and develop a symptom-based risk score useful in primary healthcare. Study design and setting: 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. Results: 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 <20 and ≥80 years vs. those aged 40-59, 1.27 (1.03-1.55) for smokers vs. nonsmokers, and 1.91 (1.59-2.29) for individuals without vs. with case contact. In symptomatic population, a symptom-based score (weights: severe tiredness = 1; absence of sore throat = 1; fever = 2; anosmia/ageusia = 5) reached standardized seroprevalence ratio of 8.71 (7.37-10.3), discrimination index of 0.79 (0.77-0.81), and sensitivity and specificity of 71.4% (68.1-74.4%) and 74.2% (73.1-75.2%) for a score ≥3. Conclusion: The presence of anosmia/ageusia, fever with severe tiredness, or fever without sore throat should serve to suspect COVID-19 in areas with active viral circulation. The proportion of asymptomatics in children and adolescents challenges infection control.engVoRhttp://creativecommons.org/licenses/by-nc-nd/4.0/COVID-19General populationNationwide serosurveyPrediction modelSARS-CoV-2 infectionSymptomsAttribution-NonCommercial-NoDerivatives 4.0 Internacional34126206139240-25410.1016/j.jclinepi.2021.06.0051878-5921Journal of Clinical Epidemiologyopen access