[Effectiveness of COVID-19 mRNA vaccine against hospitalization during the alpha and delta variant surges in Tokyo].

Objective　Evidence on COVID-19 vaccine effectiveness outside of clinical trials is insufficient. We aimed to determine the real-world effectiveness of mRNA vaccines in preventing hospitalization via data from the Health Center Real-time Information-sharing System on COVID-19 (HER-SYS), a national public database on COVID-19 cases in Japan.Methods　This case-control study targeted residents of Minato-ward, Tokyo, aged ≥50 years, who were COVID-19 positive between May 17 and Sep 30, 2021 (the alpha and delta strains-dominant period). Those hospitalized within 10 days of onset or diagnosis were considered the case group and control patients were not hospitalized. Patients were grouped according to vaccination status; group 1, unvaccinated, groups 2 and 3, who received the first dose ≤14 days and ≥15 days, respectively, and groups 4 and 5, who received the second dose ≤14 and ≥15 days before onset, respectively. To estimate vaccine effectiveness, adjusted odd ratios (OR) were calculated for each group against group 1. Furthermore, to determine other risk factors for hospitalization, OR were calculated for age, sex, and presence of any underlying diseases.Results　This study analyzed 192 case and 366 control patients. The adjusted OR were 1.48 (95% Cl=0.88-2.50), 0.71 (95% Cl=0.27-1.80), 0.58 (95% Cl=0.20-1.66), and 0.30 (95% Cl=0.13-0.67) for groups 2-5, respectively. Additionally, the adjusted OR were 1.57 (95% Cl=1.07-2.29), 1.05 (95% Cl=1.03-1.07), and 1.69 (95% Cl=1.15-2.48) for presence of underlying disease, 1-year increase of age, and men, respectively.Conclusion　Patients aged ≥50 years who received the second dose of the mRNA vaccine ≥15 days before onset had a significantly lower risk of hospitalization. Additionally, older age, men, and presence of underlying diseases were risk factors for hospitalization. Further studies on vaccine effectiveness to prevent severe diseases, hospitalization, and death following booster immunization during the omicron strain-dominant period are warranted.