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

Objectives 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.