Objectives: Assess the effectiveness of influenza vaccination in reducing hospitalization due to pneumonia and influenza among elderly subjects in a community in central Italy. Estimate the hospitalization fraction preventable by extending the vaccination program.
Methods: Case-control study. Cases were subjects aged 65+ at hospital admission (1 December 1994-31 March 1995). For each case two population controls were randomly chosen, matched by sex, age and residence. Variables of interest were recorded through a postal questionnaire and telephone interview. A matched-set analysis was carried out adjusting for concomitant chronic diseases, education, type of home heating, and smoking habits. The preventable fraction of hospitalization was computed through the application of the attributable risk estimate. The setting was 33 municipalities in central Italy including 169,370 residents aged 65 years or more.
Results: Two hundred and seventy-five cases 550 controls were analyzed. Influenza vaccination was effective in preventing 33% of hospitalization due to pneumonia/influenza. The fraction of hospital admissions preventable by extending the vaccination was 17%. When the analysis was limited to self-respondents to the questionnaire (excluding next-of-kin) and to pneumonia/influenza as primary discharge diagnosis, protection from hospitalization by vaccination almost reached 50%, a better result in comparison with most case-control studies.
Conclusions: Influenza vaccination was shown to be successful in reducing hospital admissions due to pneumonia and influenza. A large number of hospitalizations could be reduced extending the vaccination campaign.