Objective : To evaluate the difference in utilization of influenza-related outpatient visits of the community-dwelling elderly between recipients and non-recipients of influenza vaccination using health insurance claims with the main purpose of appraising the effectiveness of a community-wide vaccination program
Design : A retrospective observational study
Methods : Outpatient health insurance claims of elderly enrollees of Natori city National Health Insurance program in February 2002 were matched with the list of influenza vaccination recipients in previous year. Comparison was made between vaccinated and non-vaccinated groups with respect to the share of influenza and related respiratory diseases in aggregate number of days (= outpatient visits).
Results : Influenza accounted for 0.319% of outpatient days of the non-vaccinated group while it accounted for 0.053% of the vaccinated group, or 83.4% reduction in the aggregate number of outpatient visits purely attributable to influenza. The difference was statistically significant.
Discussions : The observed difference in utilization of outpatient visits attributable to influenza was consistent with proven vaccine effectiveness of approximately 80% established in RCTs. Due to the non-randomization nature of this method, it cannot provide a valid evaluation of vaccine effectiveness. However, the observed reduction of outpatient visits attributable to influenza will yield a reasonable appraisal method to retrospectively evaluate the efficacy of a community-wide mass vaccination program particularly in view of the possible adverse selection of vaccine recipients as evidenced by the high percent of institutionalized elderly and higher per claim cost. Retrospective analysis of health insurance claims, though not a substitution of RCTs by any means, can serve as a practical method for program appraisal where RCTs are not feasible.