Fourth mRNA vaccination increases cross-neutralizing antibody titers against SARS-CoV-2 variants, including BQ.1.1 and XBB, in a very elderly population.

The positivity rates for BQ.1.1 and XBB increased after 4th vaccination, although the titer value was lower than those of BA.5 and BA.2.75. Considering the rapid mutation of viruses and the efficacy of vaccines, it may be necessary to create a system that can develop vaccines suitable for each epidemic in consideration of the epidemic of the virus.