As a result, the true response rate cannot be determined and, despite its large size, this study sample may not reliably represent the entire populace, since the participants were self-selected volunteers rather than a random sample selected from a specific sampling frame. rapidly increasing prevalence in the Czech populace during the rising pandemic wave prior to the beginning of vaccination. The difference between our results on seroprevalence and PCR screening suggests that antibody response provides a better marker of past infection than the routine testing program. Subject terms:Viral contamination, Viral contamination Piler et al. perform 3-O-(2-Aminoethyl)-25-hydroxyvitamin D3 a longitudinal serology study of the Czech populace between October 2020 and March 2021 and describe an increase in anti-SARS-CoV-2 IgG antibodies that correlates with SARS-CoV-2 PCR screening data. They include data stratified by age, sex, body mass and history of symptoms. == Plain Language Summary == The nationwide prospective seroconversion (PROSECO) study is usually monitoring the presence of anti-SARS-CoV-2 IgG antibodies in the Czech populace. The presence of these antibodies is usually indicative of a previous contamination with SARS-CoV-2. Here, we report initial results from 30,054 participants in this study that were recruited between October 2020 and March 2021, during the second wave of the COVID pandemic. The data indicates that this percentage of the population infected with SARS-CoV-2 is usually higher than estimates based on recognized data on cumulative PCR screening for presence of the computer virus. At least one third of participants with antibodies did not have symptoms of COVID and 28% had not undergone PCR screening. This study demonstrates the importance of monitoring the presence of anti-SARS-CoV-2 IgG antibodies to accurately assess the proportion of the population that has been infected over time. == Introduction == The COVID-19 pandemic is usually caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)1. This computer virus stimulates a rapid seroconversion to IgG antibodies in symptomatic2,3as well as asymptomatic subjects4,5. Therefore, the memory IgG antibodies against SARS-CoV-2 can serve as a specific long-term biomarker of previous SARS-CoV-2 contamination6as well as an appropriate marker for monitoring the persistence of antibodies against SARS-CoV-2 after vaccination7. The SeroTracker dashboard, which systematically monitors and synthesises findings from hundreds of global SARS-CoV-2 serological studies8, shows a lack of nationwide population-based seroprevalence studies in Central and Eastern Europe, compared to several seroprevalence studies 3-O-(2-Aminoethyl)-25-hydroxyvitamin D3 in other parts of Europe911. While some existing studies have examined general populace samples (ranging between 95912and 105,65113participants), most have been restricted to hotspots1417. These studies adopted different sampling and analytical strategies: some have used household and community sampling9,17while others focused on biosamples from blood donors18or residual sera in laboratories10,12. While the majority of these studies employed serological assessments, point-of-care antibody assays with lower sensitivity and specificity were also used13,19. A review by Rostamiet al. reported seroprevalence for Europe between 0.66% and 5.27% until August 202020. Thus far, only a small number of nationwide population-based studies have been published covering the period between November 2020 and March 2021, i.e. the second wave of the epidemic in Europe2123. In the Czech Republic, the first COVID-19 cases were confirmed on 1 March 2020. The first epidemic wave in spring 2020 was relatively modest, with only 9301 3-O-(2-Aminoethyl)-25-hydroxyvitamin D3 cases recorded by the end of May 2020 (86.9 confirmed cases per 100,000 persons over three months). The governmental restrictions were dismantled in the summer; this resulted in a worsening of the Eptifibatide Acetate epidemic situation in autumn 2020. At the peak of the second wave, over 10,000 new cases were diagnosed every day (over 100 confirmed cases per 100,000 persons daily) and the Czech Republic ranked among the countries with the greatest burden of COVID-19 in Europe and in the world. As of 31 May 2021, the Czech Republic experienced a cumulative total of 1 1,661,787 cases confirmed (15,528 per 100,000 persons) and the highest cumulative incidence of COVID-19 among countries with 1,000,000 inhabitants24. To address the lack of regional data and to investigate the dynamics of seroprevalence of anti-SARS-CoV-2 IgG antibodies in the region, a nationwide prospective seroconversion study (PROSECO) was initiated in the Czech.