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PurposeIn a country-wide seroprevalence study of COVID-19 in Estonia we aimed to determine the seroprevalence and the dynamics of IgG against SARS-CoV-2 after vaccination or positive PCR-test.MethodsLeftover blood samples were selected between February 8 to March 25, 2021, by SYNLAB Estonia from all counties and age groups (0-9, 10-19, 20-59, 60-69, 70-79, 80-100 years) proportionally to the whole Estonian population and tested for IgG against SARS-CoV-2 spike protein receptor-binding domain (anti-S-RBD IgG) using Abbott SARS-CoV-2 IgG II Quant assay. Antibody levels after positive PCR-test or vaccination were described by nonlinear model.ResultsA total of 2517 samples were tested. Overall seroprevalence (95% CI) was 20.1% (18.5-21.7%), similar in all age groups. If all individuals vaccinated with the first dose at least 14 days before antibody measurement were assumed to be seronegative, the overall seroprevalence was 15.8% (14.4-17.3%), 4-fold larger than the proportion of confirmed COVID-19 cases. According to nonlinear models, age increased anti-S-RBD IgG production after positive PCR-test but decreased after vaccination. The peak of anti-S-RBD IgG in a 52-year-old (median age of PCR-positive and/or vaccinated individuals) was significantly higher after vaccination compared with positive PCR-test (22082 (12897…26875) vs 6732 (2321…8243) AU/mL), but half-life was similar (26.5 (6.9…46.1) vs 38.3 (8.2…68.5) days).ConclusionOne year after the start of COVID-19 pandemic the actual prevalence of infection is still underestimated compared with confirmed COVID-19 cases, underlining the importance of seroepidemiological studies. Older individuals have lower anti-S-RBD IgG level after vaccination, but similar decline rate to younger.

The SARS-CoV-2 mRNA vaccine BNT162b2 (Pfizer-BioNTech) has become a game-changer in the COVID-19 crisis. Faster and wider coverage of vaccine supply is urgently needed, although vaccine supply is far from abundant in many countries and regions. There is no denying that Japan is vaccinating at a slower pace than in many other countries. At the end of April 2021, it had administered less than 2% of the population, according to the statistics website Our World in Data.1 Single-dose vaccination may be considered to induce an adequate antibody response in individuals with prior infection,2–5 which can lead to more effective vaccine distribution to people in serious need. However, multiple antibody responses in populations with various backgrounds, including prior COVID-19 infection, underlying diseases, and age, have not been investigated sufficiently. Due to the lack of antibody measurement data, antibody follow-up measurement recommendations have yet to be suggested. Accumulation of available data regarding quantitative antibody titer will lead to the establishment of personally customized schedules, including antibody follow-up, and will balance both faster delivery of vaccines and confirmation of effective vaccination.