INFLUENZA VACCINE EFFECTIVENESS IN PREVENTING HOSPITALIZATION AMONG THE ELDERLY DURING INFLUENZA TYPE-A AND TYPE-B SEASONS

Background. Influenza vaccine effectiveness evaluations were carried o ut among the elderly, as part of a demonstration established to estima te the value of including influenza vaccination as a covered Medicare benefit. Methods. Cases hospitalized with pneumonia and influenza-rela ted diagnoses during November through April were identified and group matched to randomly selected community controls. Data were collected f rom cases and controls on influenza vaccination status and other facto rs which could have confounded the association between vaccination and hospitalization. A community-based influenza surveillance programme w as conducted each year to determine the timing and aetiology of influe nza activity. Logistic regression analyses were carried out to evaluat e the association of influenza vaccination with the likelihood of hosp italization. Results. In 1990-1991, during the peak of the influenza t ype B outbreak, influenza vaccination was estimated to be 31% (95% CI : 4-51%) effective in reducing the likelihood of hospitalization. In 1 991-1992, during the peak of the influenza type A(H3N2) epidemic, a ne arly identical point estimate for vaccine effectiveness was demonstrat ed (32%, 95% CI : 7-50%). Identical analyses carried out each year dur ing the periods of low or absent influenza activity failed to demonstr ate a significant effect for vaccination in preventing hospitalization . Conclusion. Results indicated that a significant benefit for vaccina tion could be expected during both type A and type B influenza seasons .