Dynamics of Seroconversion of Anti-Sars-Cov-2 Igg Antibodies in the Czech Unvaccinated Population: Nationwide Prospective Seroconversion (Proseco) Study

Background: Although the Czech Republic weathered the first wave of the COVID-19 epidemic with relatively low incidence, the second wave of the global pandemic saw it rank among countries bearing the greatest COVID-19 burden, both in Europe and on a worldwide scale. The aim of the nationwide prospective seroconversion (PROSECO) study was to investigate the dynamics of seroconversion of anti-SARS-CoV-2 IgG antibodies in the Czech population. Methods: All clients of the second largest health insurance company in the Czech Republic were sent a written invitation to participate in this longitudinal study. The study includes the first 30,054 persons who provided a blood sample between October 2020 and March 2021. Seroprevalence was compared between calendar periods of blood sample collection, RT-PCR test results, sociodemographic factors, and other characteristics. Findings: The data show a dramatic increase in seropositivity over time, from 28% in October/November 2020 to 43% in December 2020/January 2021 to 51% in February/March 2021. These trends were consistent with government data on cumulative viral antigenic prevalence in the population captured by PCR testing – although the seroprevalence rates established in this study were considerably higher than those listed in government data. Data pooled across the entire study period exhibited minor differences in seropositivity between sexes, age groups and body mass index categories; results were similar between test providing laboratories. Seropositivity was substantially higher among symptomatic vs. asymptomatic persons (76% vs. 34%). At least one third of all seropositive participants were asymptomatic, and 28% participants who developed antibodies against SARS-CoV-2 never underwent PCR testing. Interpretation: Antibody response provides a better marker of past SARS-CoV-2 infection than PCR testing data. Our data on seroconversion confirm the rapidly increasing prevalence in the Czech population during the dramatically rising pandemic wave prior to the beginning of massive vaccination. The planned second and third assessment of the study participants (April 2021 – September 2021, October 2021 – March 2022) will provide valuable evidence on the seroprevalence changes following vaccination and persistence of antibodies resulting from natural infection and vaccination. Funding: Prevention Programme of the Health Insurance Company of the Ministry of the Interior of the Czech Republic, Ministry of Education, Youth and Sport of the Czech Republic, EU Horizon 2020, European Structural and Investment Funds. Declaration of Interest: We declare no competing interests. Ethical Approval: This study received approval from the ELSPAC ethics committee under reference number (C)ELSPAC/EK/5/2021.