LAUSR

OBJECTIVES to present an evaluation of the campaign for vaccination against COVID-19 in the territory covered by the Agency for Health Protection of the Metropolitan Area of Milan from 01.01.2021 to 30.09.2021. DESIGN descriptive study of vaccine adherence; predictive study of the factors associated with vaccine adherence, efficacy of vaccination in terms of hospitalization and mortality, and factors that increase the risk of hospital admission following full vaccination. SETTING AND PARTICIPANTS population-based study with subjects aged >18 years eligible for vaccination (N. 2,981,997). An information system obtained by integrating various administrative healthcare sources made it possible to analyse socioeconomic characteristics, COVID-19 related hospitalizations, and general mortality in subjects eligible for vaccination. MAIN OUTCOME MEASURES full vaccination (2 doses); COVID-19-related hospitalizations, COVID-19-related hospitalizations occurring more than 15 days after the second dose, general mortality. RESULTS in the first nine months of the vaccination campaign, 74.7% of the subjects (N. 2,228,915) was fully vaccinated, whereas 15.6% (N. 465,829) did not even receive one dose. Women have a lower probability of getting vaccinated than men; the 50-59 years and 70+ years age groups emerge as the most problematic to reach, while the younger one (<40) is the most adherent. A social gradient emerged, with residents of more disadvantaged areas progressively less incline to get vaccinated than those living in more affluent areas. Adherence is greater in Italian citizenship and is likely to increase with an increase in the number of chronic conditions. Hospitalizations amounted to 1.22% (N. 5,672) in the unvaccinated population compared to 0.05% (N. 1,013) in the vaccinated population; general mortality was 4.51% (N. 15,198) in the unvaccinated population against 0.32% (N. 8.733) in the vaccinated population. Sociodemographic factors and the presence of previous health conditions are important predictors of hospitalization outcomes even within the fully vaccinated population. Specifically, the highest hazard ratios are found in subjects with heart failure (HR 2.15; 95%CI 1.83-2.53), in immunocompromised patients (HR 2.02; 95%CI 1.52-2.69), and in transplant recipients (HR 1.92; 95%CI 1.10-3.33). CONCLUSIONS vaccination campaign adherence is affected by the sociodemographic characteristics of the population and is a determining factor in preventing hospitalizations for COVID-19 and death. The persistent higher risk of hospitalization in chronic subjects following the second dose emphasizes the need to direct booster doses to the more vulnerable. Information systems proved to be effective monitoring tools in the absence of specific trials.