[Effect of the prior influenza vaccination on serum antibody titer induction by subsequent inactivated influenza vaccine in the elderly].

In order to investigate the effects of prior influenza vaccination on subaequent annual influenza vaccination in the geriatric population, we analyzed serum hemagglutinine inhibition antibody tirers (HI titer) before and after vaccination with inactivated influenza vaccine in elderly inpatients. A total of 163 inpatients of 60 years or older were enrolled with informed consent. They were classified by vaccination status in the previous year, 53 patients had inactivated vaccine (inactivated). 52 patients had genetically assorted cold-adapted influenza live attenuated vaccine (cold-adapted), and 53 had no influenza vaccine history during the past year. The HI titer was higher in the inactivated group than in the cold-adapted and non-vaccinated groups, suggesting residual immunological effects of inactivated influenza vaccine from the previous year vaccination. The HI titer after the inactivated vaccine in 1993 was higher in both the inactivated and cold-adapted groups than in the non-vaccinated group. The number of patients with HI titers of 2(7) or higher, which is the putative protective HI titer level for influenza infection, was significantly higher in both the inactivated and cold-adapted groups than the non-vaccinated group. These results suggest that continuous annual influenza vaccination does not impair the effects of vaccination, and may actively promote elevated HI titers.