Low prevalence of active COVID-19 in Slovenia: a nationwide population study on a probability-based sample

Vodičar, Polona Maver; Valenčak, Anja Oštrbenk; Zupan, Blaž; Županc, Tatjana Avšič; Kurdija, Slavko; Korva, Miša; Petrovec, Miroslav; Demšar, Janez; Knap, Nataša; Štrumbelj, Erik; Vehovar, Vasja; Poljak, Mario

OBJECTIVES: Accurate population-level assessment of the COVID-19 burden is fundamental for navigating the path forward during the ongoing pandemic, but current knowledge is scant. We conducted the first nationwide population study using a probability-based sample to assess active SARS-CoV-2 infection, combined with a longitudinal follow-up of the entire cohort over the next 6 months. Baseline SARS-CoV-2 RNA testing results and the first 3-week follow-up results are presented. METHODS: A probability-based sample of the Slovenian population (N = 2.1 million) was selected from the Central Population Register (n = 3,000). SARS-CoV-2 RNA was detected in nasopharyngeal samples using the cobas 6800 SARS-CoV-2 assay. Each participant filled in a detailed baseline questionnaire with basic sociodemographic data and current and past medical history compatible with COVID-19. After 3 weeks participants were interviewed for the presence of COVID-19–compatible clinical symptoms and signs, including in household members, and offered immediate testing for SARS-CoV-2 RNA if indicated. RESULTS: 1,368 (46%) individuals consented to participate and completed the questionnaire. Two of 1,366 participants tested positive for SARS-CoV-2 RNA (prevalence 0.15%; posterior mean 0.18%, 95% Bayesian CI 0.03–0.47%; 95% HDR 0.01–0.41%). No newly diagnosed infections occurred in the cohort during the first 3-week follow-up round. CONCLUSIONS: The low prevalence of active COVID-19 infections found in this study accurately predicted the dynamic of the epidemic in Slovenia over the subsequent month. Properly designed and timely executed studies using probability-based samples combined with routine target-testing figures provide reliable data for informed decisions on relaxing or strengthening mitigation strategies.