The efficacy of influenza vaccine among geriatric inpatients

To investigate the efficacy of influenza vaccine in the elderly, hemagglutination inhibition (HI) antibody titer for the three types of influenza viruses were measured and the influenza infection rate was determined serologically in geriatric inpatients. Influenza vaccination was done for inpatients. For patients who had influenza vaccination in the year prior to the study, influenza vaccine was administered once or twice, and the number of injections were determined randomly. Influenza vaccine was injected twice to those had not received influenza vaccine in the previous year. Serum samples were collected from 166 vaccinated and 104 unvaccinated patients before and after 1996/1997 influenza season. In the vaccinees who had been vaccinated the previous year, 56 patients were injected once and 58 patient were injected twice. Fifty-two patients had not been vaccinated the previous year. Serologically diagnosed influenza infection rate in the 104 unvaccinated patients was 16.3% for influenza A/H3N2 and 8.7% for influenza B. The infection rate was 3.0% for influenza A/H3N2 and 0.6% for influenza B in the 166 vaccinated patients. The infection rates were significantly lower in the vaccinees than in the unvaccinated patients (p < 0.001 with A/H3N2 and p < 0.01 with B). There was no significant difference in the infection rate among the three vaccinated groups. These results suggest that the influenza vaccination had significant protective efficacy for influenza infection in the elderly. Prior vaccination did not diminish the efficacy of the influenza vaccine. The efficacy of a single influenza vaccine injection was equivalent to that of two injection.