The 1 st year of the COVID-19 epidemic in Estonia: a population-based nationwide sequential/consecutive cross-sectional study

The objective of this study was to assess the population prevalence of SARS-CoV-2 and changes in the prevalence in the adult general population in Estonia during the 1 year of COVID-19 epidemic. This was a population-based nationwide sequential/consecutive cross-sectional study. Using standardised methodology (population-based, random stratified sampling), 11 cross-sectional studies were conducted from April 2020 to February 2021. Data from nasopharyngeal testing and questionnaires were used to estimate the SARS-CoV-2 RNA prevalence and factors associated with test positivity. Between April 23, 2020, and February 2, 2021, results were available from 34,915 individuals and 27,870 samples from 11 consecutive studies. The percentage of people testing positive for SARS-CoV-2 decreased from 0.27% (95% confidence interval [CI] = 0.10%-0.59%) in April to 0.04% (95% CI = 0.00%-0.22%) by the end of May and remained very low (0.01%, 95% CI = 0.00%-0.17%) until the end of August, followed by an increase since November (0.37%, 95% CI = 0.18%-0.68%) that escalated to 2.69% (95% CI = 2.08%-2.69%) in January 2021. In addition to substantial change in time, an increasing number of household members (for one additional odds ratio [OR] = 1.15, 95% CI = 1.02-1.29), reporting current symptoms of COVID-19 (OR = 2.21, 95% CI = 1.59-3.09) and completing questionnaire in the Russian language (OR 1.85, 95% CI 1.15-2.99) were associated with increased odds for SARS-CoV-2 RNA positivity. SARS-CoV-2 population prevalence needs to be carefully monitored as vaccine programmes are rolled out to inform containment decisions.