Temporal Surveillance of the Humoral Immunity against Influenza Vaccine in the Elderly over 9 Consecutive Years

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Abstract
Background: The effect of age on declining immunity is well established, but its influence on humoral responses to influenza vaccines is still debated. Objective: To compare prospectively the immunogenicity of an influenza vaccine in elderly and healthy control subjects and to search for correlations between this specific humoral protection and clinical and virological parameters. Methods: A prospective study of the humoral response to influenza vaccine was conducted over 9 consecutive years in the long-term care units of the Saint-Etienne University Hospital, France. Antibodies directed against the vaccinal strains in an inactivated trivalent vaccine were tested by radial haemolysis before and 1 month after 477 and 242 cumulative annual vaccinations in elderly people and in healthy controls, respectively. During the last 6 years of the study, clinical samples from patients with fever ≧38°C were tested systematically for the diagnosis of an influenza infection. Results: A significant rise in anti-A/H3N2 antibody titres was observed in 49.3% of the elderly subjects and in 48.3% of the controls (not significant). The rises for the A/H1N1 and B strains were 31.4 and 39.2% (p < 0.001) and 30.6 and 40.5 (p < 0.001), respectively. When control subjects under 31 years old were excluded, no significant difference was recorded for any strain. Only 3.9% of the vaccinated elderly experienced any clinical influenza infection as defined by fever ≧38°C and virological proof. Conclusions: The humoral response to influenza immunization is not impaired in the elderly compared with middle-aged controls, and the incidence of febrile episodes due to influenza is low in this vaccinated elderly population. These findings provide further evidence supporting the recommendation for annual influenza vaccination in the elderly.