Abstract
COVID-19 vaccines reduce hospitalization risk, but data on severe outcomes are limited. We analyzed the impact of COVID-19 vaccination on severe outcomes in hospitalized patients in Hungary during the pre-Omicron era, addressing a regional knowledge gap. This retrospective study included hospitalized patients with PCR-confirmed COVID-19 (March 2020 - December 2021) who were categorized as unvaccinated, primary immunized, or booster-vaccinated. Outcomes included oxygen therapy, ventilation types, ECMO, and death, with the most severe outcome as the primary outcome and individual outcomes as secondary measures. Polytomous logistic regression calculated relative risk ratios for the primary outcome and COVID-19 vaccination status, while logistic regression estimated odds ratios for individual outcomes. During the study, 7575 patients were hospitalized with PCR-confirmed COVID-19: 6420 (84.8 %) were unvaccinated, 1016 (13.4 %) received a primary vaccination series, and 139 (1.8 %) had received a booster dose. COVID-19 vaccination reduced the risk of both invasive ventilation and in-hospital death as the most severe outcome by 50 % within 12 months (relative risk ratio [RRR]: 0.52, 95 % CI: 0.30-0.89; 0.50, 95 % CI: 0.41-0.61). Booster doses within six months decreased the risk of in-hospital death to a similar extent (RRR 0.46, 95 % CI: 0.30-0.72). Primary and booster vaccination reduced the risk of progression to severe outcomes in hospitalized COVID-19 patients.