COVID-19 vaccination benefits in preventing severe disease in mild-to-moderate cases: An analysis in the first specialized hospital for COVID-19 in Japan

BACKGROUND: In Japan, the fourth round of coronavirus disease (COVID-19) vaccination is ongoing and is targeted at medical staff and nursing home workers, individuals aged ≥60 years, and those with comorbidities or other high-risk factors, including body mass index (BMI) ≥30 kg/m(2). The incidence of severe COVID-19 decreased markedly after widespread COVID-19 vaccination drives, and our hospital experienced a similar trend. We, therefore, examined the characteristics of our patients to clarify who benefited the most from vaccination. METHODS: We retrospectively investigated all patients hospitalized for COVID-19 in Osaka City Juso Hospital between March 1, 2021, and June 30, 2022. Using multivariable logistic analysis, we calculated the adjusted odds ratios (aORs) for severe disease after vaccination in the whole dataset and in subsets stratified by age, sex, BMI, smoking history, pre-hospitalization location, and comorbidities. RESULTS: The analysis included 1041 patients. Multivariable logistic analysis showed that vaccination was associated with a low risk of severe disease, with an aOR of 0.21 (95% confidence interval: 0.12–0.36, p < 0.001). On stratifying the analysis according to background characteristics, lower aORs for severe COVID-19 were found for patients aged ≥60 years and for those with diabetes or hypertension. Notably, patients with BMI >30 kg/m(2) and those with BMI ≥18 kg/m(2) and ≤30 kg/m(2) benefited from vaccination. CONCLUSIONS: Individuals with diabetes or hypertension and those of age ≥60 years benefited more from vaccination than did their counterparts. We recommend extending the fourth round of vaccinations to individuals with a BMI of 18–30 kg/m(2).