Effectiveness of vaccination on hospitalisation related outcomes: a population-cased study in Northern Italy

Background and objective: Although the efficacy of SARS-CoV-2 vaccines in preventing severe disease – especially in older adults – is widely demonstrated, there are not many studies conducted in the Alps with large sample sizes and during Omicron variant period. Moreover, scarce literature analyses the hospitalisation length of SARS-CoV-2 patients and its association with vaccination status. Aim of this study is to investigate the associations between ordinary or intensive care unit (ICU) hospitalisations and the vaccination status of SARS-CoV2 adult patients resident in the Italian alpine province of Bolzano. Methods: We collected anonymized data from 93643 patients who swabbed positive for SARS-CoV-2 between November‘21 and February’22 – coinciding with the arrival of the Omicron variant. Associations between vaccination status and hospitalisation characteristics were assessed using bivariate statistics and multiple logistic regression. Results: Of 93643 positive patients, 925 were hospitalised (90% ordinary ward, 10% ICU). Among over 65 (8% of the sample and 68% of hospitalisations), vaccinated patients had a significantly lower risk of being admitted to ordinary wards compared to not vaccinated: Odds Ratio(OR)=0.40(CI:0.35-0.48); to ICU: OR=0.26(CI:0.15-0.47) and to die: OR=0.39(CI:0.29-0.52). These risks were even lower for vaccinated with booster compared to unvaccinated. Suggestively, also among vaccinated patients those with booster showed decreased risk of hospitalisation: OR=0.54(CI:0.42-0.67); and death: OR=0.39(CI:0.25-0.61). Similar associations were found in age range 30-65. Finally, average duration of ICU stay was significantly longer for unvaccinated people compared to vaccinated (9-6 days; p<0.003), especially in over 65 (11-6 days; p<0.05). Conclusion: Results suggest that vaccination played a protective role against severe disease, even during Omicron variant period and especially in the most at-risk age groups. Additional/booster dose have granted a greater protection compared to the primary cycle, supporting its administration policy. The shorter ICU stay of vaccinated people suggests that vaccination could have made recovery faster