Influenza vaccine effectiveness for the elderly: a cohort study involving general practitioners from Abruzzo, Italy

In all Italian regions influenza vaccine is routinely administered to the elderly population. However, vaccination impact has been rarely evaluated because of the high costs of conventional cohort investigations. A promising low-cost alternative approach uses administrative discharge data to derive vaccine effectiveness indicators (hospitalizations and/or deaths) and involves General Practitioners (GPs) to document the exposure. We conducted a cohort analysis using such approach to assess influenza vaccine effectiveness and to investigate the feasibility and validity of that methodology for routine vaccine evaluation.

During October 2006, all GPs from two Local Health Units (LHUs) were requested to indicate immunization status of all their patients in a specific form containing patient's demographic records. Immunization status information were also collected from Prevention Departments. Main outcomes were hospitalizations for influenza and/or pneumonia. Analyses were based upon random-effect logistic regression.

Of a total of 414 GPs assisting 103,162 elderly, 116 GPs (28%) provided data on 32,457 individuals (31.5%). The sample was representative and had an overall 66.2% vaccina-tion rate. During the first semester 2007, the hospitalization rate was low in the sample, with only 7 elderly patients admitted for influenza and 135 for pneumonia. At either bivariate or multivariate analysis, vaccination did not significantly reduce the risk of in-hospital death, influenza or pneumonia admission.

The study had minimal costs, recruited a large and representative sample size, and had no evidence of a substantial selection bias. Administrative and GP's data may be successively pooled to provide routine assessment of vaccination effectiveness.