Clinical characteristics and outcomes of vaccinated patients hospitalised with SARS-CoV-2 breakthrough infection: Multi-IPV, a multicentre study in Northern Italy

Background: Despite the well-known efficacy of anti-COVID-19 vaccines in preventing morbidity and mortality, several vaccinated individuals are diagnosed with SARS-CoV-2 breakthrough infection, which might require hospitalisation. This multicentre, observational, and retrospective study aimed to investigate the clinical characteristics and outcomes of vaccinated vs. non -vaccinated patients, both hospitalised with SARS-CoV-2 infection in 3 major hospitals in Northern Italy. Methods: Data collection was retrospective, and paper and electronic medical records of adult patients with a diagnosed SARS-CoV-2 infection were pseudo-anonymised and analysed. Vaccinated and non -vaccinated individuals were manually paired, using a predetermined matching criterion (similar age, gender, and date of hospitalisation). Demographic, clinical, treatment, and outcome data were compared between groups differing by vaccination status using Pearson's Chi-square and Mann -Whitney tests. Moreover, multiple logistic regression analyses were performed to assess the impact of vaccination status on ICU admission or intra-hospital mortality. Results: Data from 360 patients were collected. Vaccinated patients presented with a higher prevalence of relevant comorbidities, like kidney replacement therapy or haematological malignancy, despite a milder clinical presentation at the first evaluation. Non -vaccinated patients required intensive care more often than their vaccinated counterparts (8.8% vs. 1.7%, p = 0.002). Contrariwise, no difference in intra-hospital mortality was observed between the two groups (19% vs. 20%, p = 0.853). These results were confirmed by multivariable logistic regressions, which showed that vaccination was significantly associated with decreased risk of ICU admission (aOR=0.172, 95%CI: 0.039-0.542, p = 0.007), but not of intra-hospital mortality (aOR=0.996, 95%CI: 0.582-1.703, p = 0.987). Conclusions: This study provides real -world data on vaccinated patients hospitalised with COVID-19 in Northern Italy. Our results suggest that COVID-19 vaccination has a protective role in individuals with higher risk profiles, especially regarding the need for ICU admission. These findings contribute to our understanding of SARS-CoV-2 infection outcomes among vaccinated individuals and emphasise the importance of vaccination in preventing severe disease, particularly in those countries with lower first -booster uptake rates.