Association of COVID-19 vaccine intake with diagnosis, hospitalization, and oxygenation/ventilation: A longitudinal analysis, 2021-2022, Japan

Limited national-level data exist on COVID-19 vaccine effectiveness against infection outcomes based on individual characteristics. We analyzed 19,482 individuals aged 16-81 who responded to baseline (2021) and follow-up (2022) Internet-based surveys. COVID-19 vaccine intake (0/1/2+ doses) during the follow-up period was examined, and outcomes included COVID-19 diagnosis, hospitalization, and oxygenation/ventilation. Adjusted prevalence ratios (APRs) were computed using Poisson regression models, controlling for baseline characteristics including precautionary measures practiced. 81.6% of respondents received ≥1 dose of COVID-19 vaccine during the follow-up period. Among those without COVID-19 history at baseline (N=19,182), 10.9% and 6.6% reported COVID-19 diagnosis within the past year and past 2 months at follow-up, respectively. Respondents who received 1 or 2+ doses were less likely to be diagnosed in the past year (APR=0.76 and 0.43) and past 2 months (APR=0.87 [not statistically significant] and 0.51) compared to those who did not. Among 1,999 respondents diagnosed with COVID-19 during the follow-up, those with 1 or 2+ vaccine doses showed lower likelihoods of hospitalization (APR=0.78 and 0.86) and receipt of oxygenation/ventilation (APR=0.87 and 0.61), although not statistically significant. Considering the interaction of socioeconomic and behavioral characteristics, the results supported the protective effect of the COVID-19 vaccine against infection.