Objectives: We aimed to evaluate COVID-19 messenger RNA vaccine effectiveness during the Delta- and Omicron-predominant periods in Japan. Methods: We conducted a population-based cohort study among individuals aged 16–64 years during two periods: the Delta-predominant period (July 1–December 31, 2021) and the Omicron-predominant period (January 1–March 29, 2022). Results: When comparing individuals who were vaccinated with those who were unvaccinated, the effectiveness of a second dose against symptomatic infection was 89.8% (95% confidence interval [CI]: 80.5–94.7%) during the Delta-predominant period and 21.2% (95% CI: 11.0–30.3%) during the Omicron-predominant period. The effectiveness of a third dose against symptomatic infection was 71.8% (95% CI: 60.1–80.1%) during the Omicron-predominant period. Conclusion: Vaccine effectiveness against symptomatic infection decreased during the Omicron-predominant period but was maintained by a third dose.
All Science Journal Classification (ASJC) codes
- Microbiology (medical)
- Infectious Diseases