Real-world vaccine effectiveness of mRNA vaccines for SARS-CoV-2; a test-negative case-control study in a medium-sized clinic

Although nationwide immunization with SARS-CoV-2 mRNA vaccines began in February 2021, the evaluation of vaccine effectiveness (VE) using a test-negative design has not been conducted adequately in Japan. To evaluate the effectiveness of the SARS-CoV-2 mRNA vaccines, we conducted a test-negative case–control study during the periods dominated by the Delta and Omicron variants. In total, 518 and 358 adult participants with COVID-19-like symptoms were tested for the virus from August to October 2021 (Delta variant predominance) and in February 2022 (Omicron variant surge), at the Kawasaki Saiwai Clinic. During Delta variant predominance, the effectiveness of full vaccination was 90.4% (95% confidence interval [CI]: 82.1–94.8) and 97.3% (95% CI: 71.7–99.7) against all COVID-19 and moderate-to-severe disease, respectively. However, partial vaccination failed to show effectiveness against moderate-to-severe COVID-19. The effectiveness of the mRNA vaccines against all COVID-19 infection declined to 16.1% (95% CI: −81.0 to 61.1) in February 2022. Our results indicated that, although mRNA vaccines showed significant preventive effects against all COVID-19 during Delta variant predominance, these preventive effects waned during Omicron variant surge. To the best of our knowledge, this is the first study that evaluated VE in the Japanese population during both periods.