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

PurposeMonitoring 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.Methods22,130 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).ResultsBefore 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.ConclusionThe 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.