ABSTRACT
BACKGROUND: Evaluating COVID-19 vaccine effectiveness (VE) domestically is crucial for assessing and determining national vaccination policy. This study aimed to evaluate VE of mRNA COVID-19 vaccines in Japan. METHODS: We conducted a multicenter test-negative case-control study. The study comprised individuals aged ≥16 visiting medical facilities with COVID-19-related signs or symptoms from 1 January to 26 June 2022, when Omicron BA.1 and BA.2 were dominant nationwide. We evaluated VE of primary and booster vaccination against symptomatic SARS-CoV-2 infections and relative VE of booster compared with primary. RESULTS: We enrolled 7,931 episodes, including 3,055 test positive. The median age was 39, 48.0% were male, and 20.5% had underlying medical conditions. In individuals aged 16 to 64, VE of primary vaccination within 90 days was 35.6% (95% CI, 19.0-48.8%). After booster, VE increased to 68.7% (60.6-75.1%). In individuals aged ≥65, VE of primary and booster was 31.2% (-44.0-67.1%) and 76.5% (46.7-89.7%), respectively. Relative VE of booster compared with primary vaccination was 52.9% (41.0-62.5%) in individuals aged 16 to 64 and 65.9% (35.7-81.9%) in individuals aged ≥65. CONCLUSIONS: During BA.1 and BA.2 epidemic in Japan, mRNA COVID-19 primary vaccination provided modest protection. Booster vaccination was necessary to protect against symptomatic infections.