Increase in antibody titer and change over time associated with severe acute respiratory syndrome coronavirus 2 infection after mRNA vaccination: Consideration of the significance of additional vaccination.

Spontaneous infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) after mRNA vaccination causes a marked increase in antibody titer because of the combined effect of vaccine and infection ("hybrid immunity"). In this study, we discuss the significance of the mRNA vaccine booster inoculation that has been repeatedly performed in Japan. We describe the temporal trends of antibody titers in cases in which antibody titers were markedly increased by hybrid immunization. The antibody titer increased with hybrid immunization and tended to decrease with time. However, several cases maintained high antibody titers for approximately 1 year after coronavirus disease 2019 (COVID-19) diagnosis, even without booster vaccination. Most Japanese patients naturally infected with COVID-19 were infected after mRNA vaccination, and many maintained high antibody titers due to hybrid immunity. The significance of additional vaccination in hybrid-immunized cases is highly questionable regarding cost-effectiveness and risk-benefit.