Influenza vaccine effectiveness in preventing hospitalizations and deaths in persons 65 years or older in Minnesota, New York, and Oregon: Data from 3 health plans

This study developed methods and determined the impact of influenza vaccination on elderly persons in 3 large health plans: Kaiser Permanente Northwest, HealthPartners, and Oxford Health Plans. Data for the 1996-1997 and 1997-1998 seasons were extracted from administrative databases. Subjects were health plan members greater than or equal to 65 years old. Comorbid conditions collected from the preceding year were used for risk adjustment with logistic regression. The virus-vaccine match was excellent for year 1 and fair for year 2. Both years, during peak and total periods, vaccination reduced all causes of death and hospitalization for pneumonia and influenza: hospitalizations were reduced by 19%-20% and 18%-24% for years 1 and 2, respectively, and deaths were reduced by 60%-61% and 35%-39% for the same periods. These results show that all elderly persons should be immunized annually for influenza. The methods used in this study are an efficient cost-effective way to study vaccine impact and similar questions.