Prospective investigation of SARS-CoV-2 seroprevalence in relation to natural infection and vaccination between October 2020 and September 2021 in the Czech Republic

Purpose Monitoring the seroprevalence of COVID-19 antibodies is an important tool to design and adjust preventive strategies. We used prospective data on a large population sample with repeated IgG antibody measurement to examine changes in seropositivity before and during the national vaccination campaign in the Czech Republic. Methods 22,130 persons provided blood samples for COVID-19 IgG antibody measurement at two time points approximately 5-7 months apart, between October 2020 and March 2021 (Phase 1, before vaccination), and between April and September 2021 (during vaccination campaign). Results Before vaccination (Phase 1), seroprevalence increased from 15% in October 2020 to 56% in March 2021. By the end of Phase 2, in September 2021, prevalence increased to 91%; the highest seroprevalence was seen among vaccinated persons with and without previous SARS-CoV-2 infection (99.7% and 97.2%, respectively), while the lowest seroprevalence was found among unvaccinated persons with no signs of disease (26%). Vaccination rates were lower in persons who were seropositive in phase 1 but increased with age and body mass index. Only 9% of unvaccinated subjects who were seropositive in phase 1 became seronegative by phase 2. Conclusion The rapid increase in seropositivity during the 2nd wave of the COVID-19 epidemic (covered by phase 1 of this study) was followed by a similarly steep rise in seroprevalence during the national vaccination campaign, reaching seropositivity rates of over 97% among vaccinated persons. ### Competing Interest Statement The authors have declared no competing interest. ### Funding Statement the Prevention Programme of the Health Insurance Company of the Ministry of the Interior of the Czech Republic; The RECETOX Research Infrastructure, Ministry of Education, Youth and Sports of the Czech Republic (LM2018121); CETOCOEN PLUS project of ESIF (CZ.02.1.01/0.0/0.0/15\_003/0000469); European Union Horizon 2020 research and innovation programme under grant agreement No 857560 and the Ministry of Education, Youth and Sport of the Czech Republic/ESIF (CZ.02.1.01/0.0/0.0/17\_043/0009632) ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: The study, including all aspects of data collection and data analysis, was approved by the ELSPAC ethics committee under reference number (C)ELSPAC/EK/5/2021. I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines and uploaded the relevant EQUATOR Network research reporting checklist(s) and other pertinent material as supplementary files, if applicable. Yes All data generated during the first and second phases of the PROSECO study is presented in this article. Anonymised data can be made available from the corresponding author upon request once all study phases are completed and data validated. Release of data is a subject of approval of the Ethical and Scientific boards of the PROSECO study.