Duration of effectiveness of the COVID-19 vaccine in Japan: A retrospective cohort study using large- scale population-based registry data

Background Most evidence of the waning of vaccine effectiveness is limited to a relatively short period after vaccination. Methods Data obtained from a linked database of healthcare administrative claims and vaccination records maintained by the municipality of a city in the Kanto region of Japan were used in this study. The study period extended from April 1, 2020, to December 31, 2022. The duration of the effectiveness of the COVID-19 vaccine was analyzed using a time-dependent piecewise Cox proportional hazard model using the age, sex and history of cancer, diabetes, chronic obstructive pulmonary disease, asthma, chronic kidney disease, and cardiovascular disease as covariates. Results Among the 174,757 eligible individuals, 14,416 (8.3%) were diagnosed with COVID-19 and 936 (0.54%) were hospitalized for COVID-19. Multivariate analysis based on the time-dependent Cox regression model revealed a lower incidence of COVID-19 in the one-dose group (hazard ratio, 0.76 [95% confidence interval, 0.63–0.91]), two-dose (0.89 [0.85–0.93]), three-dose (0.80 [0.76–0.85]), four-dose (0.93 [0.88–1.00]), and five-dose (0.72 [0.62–0.84]) groups. A lower incidence of COVID-19-related hospitalization was observed in the one-dose group (0.42 [0.21–0.81]), two-dose (0.44 [0.35–0.56]), three-dose (0.38 [0.30–0.47]), four-dose (0.20 [0.14–0.28]), and five-dose (0.11 [0.014–0.86]) groups. Multivariable analyses based on the time-dependent piecewise Cox proportional hazard model revealed significant preventive effects of the vaccine at 0–1, 1–2, 2–3, 3–4, 7–8, ≥ 12 months for the incidence of COVID-19 and 0–1, 1–2, 2–3, 3–4, 4–5, 5–6, 7–8, and 9–10 months for hospitalization. Conclusions Vaccine effectiveness showed gradual attenuation with time after vaccination; however, protective effects against the incidence of COVID-19 and hospitalization were maintained for 4 months and ≥ 6 months, respectively. These results may aid in formulating routine vaccination plans after the COVID-19 pandemic.