Long-term effectiveness of COVID-19 vaccination in preventing hospitalization among household contacts: A case–control study from December 2021 to April 2023

We aimed to assess the long-term effect of Coronavirus disease 2019 (COVID-19) vaccination in improving infection, symptoms and disease severity more than 1 year. Consecutive adult inpatients diagnosed with COVID-19 through clinical criteria from two hospitals with household contacts were included. The exposure was COVID-19 vaccination. The primary outcome was hospitalization, while infection, symptoms, and pneumonia were assessed as exploratory outcomes. Multivariable logistic regression, subgroup and sensitivity analysis were conducted. From December 18, 2022, to April 7, 2023, 1,410 individuals were prospectively screened, and 1,047 individuals were included. The male proportion was 49.24% with the mean age of 66.10 years. A total of 883 (84.34%) participants were vaccinated with a median time of 297.5 (157.0, 389.0) days between the last vaccine and enrollment, and a median time of 446.5 (162.8, 504.8) days was found in fully vaccinated patients. The infection rate among unvaccinated patients was significantly higher (96.72% vs. 88.69%, <i>p</i> < .001). Unvaccinated patients reported less symptoms, including sore throat, myalgia and taste disorder. They had a higher hospitalization rate (80.49% vs. 45.07%). In multivariable logistic regression, vaccination was associated with reduced pneumonia risk (aOR: 0.49, 95% CI: 0.32-0.76, <i>p</i> = .002). In addition, significant negative association between vaccination and hospitalization risk (aOR: 0.37, 95% CI: 0.26-0.61, <i>p</i> < .001) was observed. For vaccination status, the booster vaccination group revealed the aOR of 0.40 (95% CI: 0.24-0.65, <i>p</i> < .001), while the fully vaccination group only presented with the aOR of 0.50 (95% CI: 0.28-0.87, <i>p</i> = .013). While COVID-19 vaccination may not provide protection against symptoms beyond 1 year, it maintains durable effectiveness in reducing the risk of hospitalization.