The 1st year of the COVID-19 epidemic in Estonia: an interrupted time series of population based nationwide cross-sectional studies.

BackgroundDecisions about the continued need for control measures and the effect of introducing COVID-19 vaccinations rely on accurate and population-based data on SARS-CoV-2 positivity and risk factors for testing positive. MethodsIn this interrupted time series of population-based nationwide cross-sectional studies, data from nasopharyngeal testing and questionnaires were used to estimate the SARS-CoV-2 RNA prevalence and factors associated with test positivity over the 1st year of the COVID-19 epidemic. The study is registered with the ISRCTN Registry, ISRCTN10182320. ResultsBetween 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% 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 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. ConclusionsSARS-CoV-2 population prevalence needs to be carefully monitored as vaccine programmes are rolled out in order to inform containment decisions. Strengths and limitations of this studyO_LIOur study relies upon nation-wide and population-based data on SARS-CoV-2 prevalence, and presents changes in prevalence over the whole 1st year of the Covid-19 epidemic. C_LIO_LIOur analysis of SARS-CoV-2 infection risk factors is not limited to notification or health care-based case data. C_LIO_LISelection bias may have been introduced as a result of low response rate. The direction of bias is unclear, but most likely operates rather uniformly over the period of observation, though this presents less of a threat to the SARS-CoV-2 prevalence trend analysis. C_LIO_LIOur data could be used to adequately project the future course of the SARS-CoV-2 epidemic and the effect of control measures. C_LI