 This study examined the effectiveness of COVID-19 vaccines by measuring antibody titers in individuals who received multiple doses of the vaccine. It found that antibodies declined over time, but those who had been infected with SARS-CoV-2 showed higher antibody titers than those who had not been infected. Additionally, the study demonstrated that the third booster vaccination was effective in increasing antibody titers. This article was authored by Hiroshi Kusunoki, Michiko Okosa, Rihita, and others.