Effects of Repeated Annual Influenza Vaccination on Antibody Responses against Unchanged Vaccine Antigens in Elderly Frail Institutionalized Volunteers

Background: Concern about the possibility that annually repeated influenza immunizationmayinduce a lower antibody response than first vaccination. Objective: To ascertain the cumulative effects of yearly vaccination on serological response to unaltered vaccine antigens in the elderly. Methods: The haemagglutination-inhibiting (HI) antibody response was examined in 158 elderly institutionalized frail volunteers subdivided in 3 groups according to the sequential winters in which each subject received a trivalent inactivated influenza vaccine. The study, conducted over 5 consecutive winters (from 1998/99 to 2002/03), reports the antibody response only for sequential years (2 or 3) in which the vaccine strain examined was not altered. Results: Significant increases in the values of HI antibody titres were observed after vaccination in each year examined against the different influenza vaccine strains used, except against B antigen in the second of the 3 winters studied (1999/00). The antibody responses found were not always adequate, i.e. at levels above the currently requested values for commercial vaccines (post-vaccination seroprotection rate ≧1:40, increases in geometric mean titres ≧2, positive responses ≧30% compared with pre-vaccination), probably because of old age (mean age ≧81 years) and the presence of underlying diseases in a high percentage of volunteers (≧86%). The most frequent chronic diseases found werecardiovascular diseases (48%), endocrine disorders (19%), functional disability (10%) and pulmonary diseases (4%). The post-vaccination values observed in the sequential years were in general similar for A/H3N2 and A/H1N1 vaccine strains. A decrease, however, for some parameters at statistically significant levels, was observed against B antigen following repeated vaccine administrations. Conclusion: Our data seem to support the possibility of a slight impairment of HI antibody response against unaltered influenza vaccine antigens, especially for influenza strains that have circulated for prolonged periods of time. Indeed a tendency to a lower response was found only against B/Beijing antigen, introduced in the vaccine composition in the winter 1995/96, but not against the A/H3N2 and A/H1N1 vaccine strains, which weremore frequently changed.