Protection conferred by booster vaccine doses in hospitalized patients with COVID-19 during the SARS-CoV-2 Omicron BA.2 and BA.5 epidemics from 2022 to 2023 in Greece

Background: To estimate the protection that coronavirus disease 2019 (COVID-19) vaccine doses conferred to hospitalized patients with COVID-19 against adverse outcomes and longer length of stay during the Omicron BA.2 and BA.5 subvariant epidemics in Greece. Methods: The study was conducted from November 2022 to May 2023. Multivariable logistic and negative binomial regression models were applied to estimate the association between any adverse outcomes and length of stay with the number of COVID-19 vaccine doses. Results: We studied 962 patients (median age: 78 years; mean length of stay: 9.2 days), of whom 847 (88.0%) had ≥ 1 comorbidity. Of these, 39 (4.0%) were admitted to the intensive care unit, 44 (4.6%) received invasive mechanical ventilation, and 110 (11.4%) died in hospital. There were 184 (19.1%) unvaccinated patients, 125 (13.0%) with one or two vaccine doses, and 653 (67.9%) with ≥ 3 doses. In multivariable analyses, patients with ≥ 3 doses had lower odds of experiencing any adverse outcomes (adjusted odds ratio: 0.57; 95% confidence interval [CI]: 0.37–0.86) compared with unvaccinated patients. On average, patients with one or two doses and those with ≥ 3 had decreased length of hospital stay (−1.5 days [95% CIs: −2.6 to −0.4] and −2.8 days [95% CIs: −4.1 to −1.4], respectively] compared with unvaccinated patients. Other characteristics consistently associated with adverse outcomes and longer length of stay included older age, having three or more comorbidities compared with none, and being admitted to the hospital two or more weeks post-diagnosis. Conclusions: A history of ≥ 3 vaccine doses conferred significant protection against any adverse outcome and longer length of stay in hospitalized patients with COVID-19.