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Protection conferred by the Omicron XBB.1.5-adapted monovalent COVID-19 mRNA vaccine to patients hospitalized with COVID-19 in Greece in 2023-2024. 2025

Helena C Maltezou, and Vasiliki Rapti, and Vasileios Petrakis, and Maria N Gamaletsou, and Evangelia Voulgaraki, and Theodoros V Giannouchos, and Eirini Antoniadou, and Konstantinos Kounouklas, and Dimitrios Basoulis, and Αmalia Karapanou, and Eleni Karantoni, and Maria Chini, and Garyfalia Poulakou, and Periklis Panagopoulos, and Dimitrios Hatzigeorgiou, and Konstantinos N Syrigos, and Nikolaos V Sipsas

Directorate for Research, Studies and Documentation, National Public Health Organization, Athens, Greece. Electronic address: maltezou.helena@gmail.com.

OBJECTIVE To estimate the protection that the Omicron XBB.1.5-adapted monovalent COVID-19 mRNA vaccine conferred to COVID-19 patients hospitalized in Greece during the 2023-2024 season. METHODS Data were collected from five tertiary-care hospitals from November 2023 through May 2024. Multivariable logistic and Poisson regression models were used to estimate the association between COVID-19 vaccination status, adverse outcome [intensive care unit (ICU) admission, invasive mechanical ventilation, death], and in-hospital length-of-stay. RESULTS All in all, 579 patients with COVID-19 [mean age: 76.7 years; 547 (94.5 %) patients with ≥ 1 comorbidity] were hospitalized for a mean 7.2 days during the study period. Overall, 111 (19.2 %) were unvaccinated, 437 (75.5 %) had been vaccinated against COVID-19 in the past, and 31 (5.3 %) had received the Omicron XBB.1.5-adapted monovalent COVID-19 mRNA vaccine. Unvaccinated individuals were disproportionately more likely to be admitted to an ICU (6.3 % versus 1.6 % versus 0.0 %; p-value = 0.01), to be intubated (6.3 % versus 1.8 % versus 0.0 %; p-value = 0.02), and to die (12.6 % versus 8.9 % versus 6.5 %; p-value = 0.42) compared to partially and fully vaccinated individuals. Multivariable analysis found that vaccination with the Omicron XBB.1.5-adapted monovalent mRNA vaccine significantly reduced the odds of in-hospital mortality [adjusted odds ratio: 0.37; 95 % confidence interval (CI): 0.15-0.90] and was associated with shorter in-hospital length-of-stay [incidence rate ratio: 0.71; 95 % CI: 0.50-0.98) compared to no vaccination. CONCLUSIONS In comparison with unvaccinated individuals, the Omicron XBB.1.5-adapted monovalent COVID-19 mRNA vaccine conferred significant protection against in-hospital mortality and reduced length of stay to COVID-19 patients hospitalized in Greece during the 2023-2024 season.