Objectives: There are no comparative studies between patients belonging to the first and second waves of the SARS-CoV-2 pandemic, the virus triggering coronavirus disease 2019 (COVID-19). In this retrospective observational study, we analyzed the clinical characteristics and the short-term outcomes of two groups of laboratory-confirmed COVID-19 patients with moderate-to-severe acute respiratory distress syndrome (ARDS) belonging to two different waves of the pandemic. Methods: We analyzed 97 consecutive patients from 11 March 2020 to 31 May 2020 and 52 consecutive patients from 28 August 2020 to 15 October 2020. Results: Patients belonging to the second wave were younger, had a lower number of concomitant chronic conditions (multimorbidity), and had a milder clinical phenotype. Medical treatments and respiratory support use have changed during the COVID-19 pandemic, based on different laboratory results and disease clinical features. Patients in the second wave had better short-term clinical outcomes, with lower death rates and more step-down transfers to a general ward. Conclusion: The present findings show a clear phenotypic difference in patients hospitalized at different stages of the COVID-19 pandemic in Italy. These results can help to stratify clinical risk and to better tailor medical treatments and respiratory support for patients with ARDS and COVID-19 pneumonia.

The association between vitamin D insufficiency and increased risk of respiratory tract infection has been widely discussed before the COVID-19 pandemic [1][2][3]. Interestingly, the

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