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Objectives: The diffusion of the SARS-CoV-2 Delta (B.1.617.2) variant and the waning of immune response after primary Covid-19 vaccination favoured the breakthrough SARS-CoV-2 infections in vaccinated subjects. To assess the impact of vaccination, we determined the severity of infection in hospitalised patients according to vaccine status., Methods: We performed a retrospective observational study on patients hospitalised in 10 centres with a SARS-CoV-2 infection (Delta variant) from July to November 2021 by including all patients who had completed their primary vaccination at least 14 days before hospital admission and the same number of completely unvaccinated patients. We assessed the impact of vaccination and other risk factors through logistic regression., Results: We included 955 patients (474 vaccinated and 481 unvaccinated). Vaccinated patients were significantly older (75.0 [63.25-84.0] vs. 55.0 [38.0-73.0]; p < 0.001), more frequently males (55.1% (261/474) vs. 46.4% (223/481); p = 0.009), and had more comorbidities (2.0 [1.0-3.0] vs. 1.0 [0.0-2.0]; p < 0.001). Vaccinated patients were less often admitted for Covid-19 (59.3% (281/474) vs. 75.1% (361/481); p < 0.001), had less extended lung lesions (≤25%: 64.3% (117/182) vs. 38.4% (88/229); p < 0.001), required oxygen less frequently (57.5% (229/398) vs. 73.0% (270/370); p < 0.001), at a lower flow (3.0 [0.0-8.7] vs. 6.0 [2.0-50.0] L/min, p < 0.001), and for a shorter duration (3 [0.0-8.0] vs. 6 [2.0-12.0] days, p < 0.001)., and required less frequently intensive care unit admission (16.2% (60/370) vs. 36.0% (133/369); p < 0.001) but had comparable mortality in bivariate analysis (16.7% (74/443) vs. 12.2% (53/433); p = 0.075). Multivariate logistic regression showed that vaccination significantly decreased the risk of death (0.38 [0.20-0.70](p = 0.002), ICU admission (0.31 [0.21-0.47](p < 0.001) and oxygen requirement (0.16 [0.10-0.26](p < 0.001), even among older patients or with comorbidities., Conclusions: Among patients hospitalised with a delta variant SARS-CoV-2 infection, vaccination was associated with less severe forms, even in the presence of comorbidities., (Copyright © 2022 European Society of Clinical Microbiology and Infectious Diseases. Published by Elsevier Ltd. All rights reserved.)