Older multimorbid frail subjects have been severely involved in the coronavirus disease-19 (COVID-19) pandemic. The aim of this retrospective study is to compare the clinical features and outcomes of patients admitted in different phases of the outbreak in a COVID-19 hospital hub, with a particular focus on age, multimorbidity, and functional dependency. The clinical records of 1264 patients with clinical and radiological features compatible with COVID-19 pneumonia admitted in February-June, 2020, were analyzed, retrieving demographical, clinical, laboratory data, and outcomes. All variables were compared after stratification by the period of admission (first phase: rising slope of pandemic wave; second phase: plateau and falling slope), age, results of the first reverse transcriptase-polymerase chain reaction (RT-PCR) test for detection of severe acute respiratory syndrome-coronavirus-2 (SARS-CoV-2), multimorbidity (≥2 chronic diseases), and presence of disability. Factors independently associated with hospital mortality were determined by multivariate forward-selection logistic regression. Patients admitted during the second phase were older, more frequently multimorbid, disabled, and of female gender. However, on admission they exhibited milder respiratory impairment (PaO₂/FiO₂ 268, IQR 174-361, vs. 238, IQR 126-327 mmHg, p < 0.001) and lower mortality (22% vs. 27%, p < 0.001). Age, respiratory exchanges, positive RT-PCR test, number of chronic diseases (odds ratio (OR) 1.166, 95% confidence interval (CI) 1.036-1.313, p = 0.011), and disability (OR 1.927, 95% CI 1.027-3.618, p = 0.022) were positively associated with mortality, while admission during the second phase exhibited an inverse association (OR 0.427, 95% CI 0.260-0.700, p = 0.001). In conclusion, older multimorbid patients were mainly hospitalized during the second phase of the pandemic wave. The prognosis was strongly influenced by the COVID-19 phenotype and period of admission, not just by age, multimorbidity, and disability.

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