Impact of Booster Vaccination on Infection and Hospitalization Rates during the Mass Vaccination in Mongolia

Objective: Our study aims to evaluate the effectiveness of booster vaccination in preventing new COVID-19 infections and related hospitalizations among groups that received the primary vaccination series. Methods: Booster vaccination started on August 31, 2021, at recruitment in May 2022. By May 2022, 64% of the Mongolian population had received two primary doses of a COVID-19 vaccine, and 31% had received a booster dose. We followed 1,251 participants (459 males and 792 females, with a mean age of 41.5 ± 14.5 years [18–93], and a median age of 39.0 years) over six months, from the start of booster immunization on September 1, 2021, to February 28, 2022. We compared infection and hospitalization rates among vaccinated groups using logistic regression and calculated vaccine effectiveness (VE) for booster recipients with the formula: VE = 1 – (vaccinated rate/unvaccinated rate) × 100. Results: All participants received two doses of one of four vaccines used in the nationwide campaign. During the study period, we identified 449 new infection cases, accounting for 35.9% of all participants, and 150 subsequent hospitalizations, or 12.0% of the total. Participants who did not receive a booster demonstrated significantly higher infection rates compared to those who did. Booster vaccination provided notably better protection against new infections among frontline workers and reduced hospitalizations in high-risk groups. No significant differences were found in VE when comparing participants based on seroconversion after initial vaccination or the type of booster vaccine used.Conclusion: Therefore, we concluded that COVID-19 booster vaccinations effectively enhanced protection against new SARS-CoV-2 infections among frontline healthcare workers, government employees, and individuals at higher risk for severe disease, thereby reducing their risk of hospitalization.