[A population study to evaluate the efficacy of influenza vaccination, 2014-2015]

to analyse the efficacy of influenza vaccination in the period 2014- 2015 in a population of a Local Health Authority of the Province of Milan (949,162 residents) in terms of reduction of short term mortality, hospital admissions, and access to emergency room.

retrospective study, based on current health databases, comparing vaccinated subjects with non-vaccinated subject with a study design based on optimal caliper widths for propensity-score matching.

74,250 subjects aged ≥65 years and 12,090 subjects aged l <65 years undergoing influenza vaccination, from November to December 2014, were compared with 74,250 and 22,090 unvaccinated subjects aged ≥65 years or <65 years, respectively (1:1).

deaths, hospitalizations in ordinary regimen, and the accesses to the emergency room for all causes and for specific causes that occurred from the inclusion in the study at 31.06.2015, with a temporal window of 6 months from vaccination, were identified.

the association between vaccination and health events was quantified by estimating hazard ratios (HR) and the corresponding 95% confidence intervals by using Cox models. For admissions and access to the emergency room, the approach based on multiple outcome according to Prentice was also used.

vaccination reduces by 34% the risk of decease (HR: 0.66; 95%CI 0.61-0.71) and about 9% of hospitalization (HR: 0.91 for first event and 0.92 for multiple events, both significant). It does not exist, however, any reduction in the risk for access to the emergency room. The analysis of all-cause and the cause-specific one yield similar results. Analysis stratified by age and number of comorbidities found that subjects aged over 65 and with at least one comorbidity shows the greater risk reduction.

influenza vaccination reduces the risk of decease and access to hospital admissions. These results, although coming from an observational study, given the need to further confirm the effectiveness of influenza vaccination and the difficulty to have results from randomised trials, must be used to reassure people of the advantage derived from some prevention practices and should open a debate on the use of current data for the rapid evaluation of interventions in public health.