Background: The effect of post-vaccination adverse events on immunogenicity is unknown. We aimed to explore relationship between post-vaccination adverse reactions and antibody levels during 6-month follow-up. Methods: Blood was serially drawn from healthcare workers after the second dose of BNT162b2 mRNA vaccine (Day 12, 30, 60, 90, 120, 150, and 180) and anti-SARS-CoV-2 spike IgG (S-IgG) levels were measured. Following each vaccine dose, volunteers completed a questionnaire regarding adverse reactions (symptomatic vs. asymptomatic groups). Results: A total of 395 subjects received the second dose of the vaccine. The main results were as follows: (i) fever after the 2nd dose was independently associated with the median S-IgG level at all follow-up time points; (ii) significantly higher S-IgG levels were observed in the symptomatic group of patients without prior COVID-19 infection throughout the entire follow-up period; (iii) prior COVID-19 positivity resulted in higher S-IgG levels only in the asymptomatic group from Day 90 of the follow-up period; (iv) both prior COVID-19 disease with asymptomatic status and symptomatic status without prior COVID-19 infection resulted in similar S-IgG antibody levels; (v) significantly lower serum S-IgG levels were observed in smokers. Conclusion: Fever may play an important role in the post-vaccination immune response in the long term.

Background: It is known that adverse reactions following SARS-CoV-2 vaccinations show a positive correlation with the subsequent antibody titer. However, it is not clear how the adverse reactions following the booster vaccination are related to the antibody levels that can be measured after a 3rd dose. The primary goal of this study was to investigate whether the adverse reactions following the booster vaccination show a correlation with subsequent antibody levels. Methods: Adverse reactions occurring within 7 days after the 3rd vaccination were recorded and the anti-SARS-CoV-2 spike protein immunoglobulin (Ig) level in the venous blood was measured on post-vaccination 14th, 60th and 120th days. Results: A total of 218 volunteers were included in the study. Main findings: (i) The adverse reactions that appeared after the booster dose did not show a positive correlation with the subsequent antibody level, except a correlation in the case of fever; (ii) there were more symptomatic patients in the group receiving heterologous booster vaccine, (iii) fever after the 2nd dose was independently associated with a reduction in the likelihood of COVID-19 positivity after the booster dose. Conclusion: No adverse reactions, but fever showed a correlation with the antibody level after the booster SARS-CoV-2 vaccine.