Introduction: 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.
Methods: 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.
Results: 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.
Discussion: 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.