Effectiveness of mRNA BNT162b2 and inactivated CoronaVac vaccines against severe COVID-19 outcomes among non-hospitalized children aged 1-3 years with SARS-CoV-2 Omicron infection

OBJECTIVES:Clinical evidence on the effectiveness of COVID-19 vaccines for children aged 1-3 years is scarce. We evaluated the effectiveness of COVID-19 vaccines among non-hospitalized children aged 1-3 years with SARS-CoV-2 Omicron infection in Hong Kong. METHODS:Our retrospective cohort of all non-hospitalized children (aged 1-3) with confirmed SARS-CoV-2 infection diagnosis between 4th August 2022 and 29th January 2023 in Hong Kong was analyzed. Vaccinated group was defined as the recipients of one or more doses of inactivated vaccine CoronaVac or mRNA vaccine BNT162b2 (original, monovalent) at least 14 days prior to the infection. Hazard ratios (HR) with 95% confidence intervals (CI) of study outcomes were estimated using Cox regression models. Effectiveness outcomes included 28-day all-cause mortality and COVID-19-related hospitalization. RESULTS:A total of 5,552 vaccinated patients and 5,552 propensity-score matched controls (unvaccinated patients) were included for analysis. The cumulative incidences of COVID-19-related hospitalization over 28 days were 2.3% and 2.9% in the vaccinated and control groups, respectively. No events of mortality were observed in both groups. COVID-19 vaccination was associated with a significant reduction in 28-day COVID-19-related hospitalization risk (HR=0.785, 95%CI=0.626-0.985, p=0.037), particularly for children aged 3 years, those who had received two or more vaccine doses, and received CoronaVac as the last dose. CONCLUSION:COVID-19 vaccination is associated with a significantly lower risk of 28-day COVID-19-related hospitalization among infected children aged 1-3 years, especially those who had received two or more doses emphasizing the importance of completing the full two-dose or three-dose series to optimize vaccine effectiveness.