Influenza vaccination in older patients. Immunogenicity, epidemiology and available agents.

Excess hospitalisation and deaths attributable to influenza virus infections often occur during epidemics and even in interepidemic periods. Influenza vaccines in current use are inactivated preparations that contain 15 micrograms each of the most recently circulating influenza A (H3N2 and H1N1) and B viruses. At present, 3 types of inactivated influenza virus vaccines are available: (a) whole virus vaccines; (b) split virus vaccines; and (c) subunit vaccines. All 3 types are similarly immunogenic in primed patients. Vaccine efficacy depends on a close antigenic match between the vaccine composition and the influenza strains circulating in the human population. The continuous antigenic drift of the viral membrane antigens (haemagglutinin and neuraminidase) necessitates an update of the vaccine composition each year according to the recommendations of the World Health Organization (WHO). Subunit and split virus vaccines cause fewer systemic reactions than whole virus vaccines. At present, live attenuated influenza virus vaccines are not licensed. In perspective, combined administration of live and inactivated vaccines seems to be advantageous. Influenza vaccine is approximately 75% effective in reducing deaths in elderly and high risk persons. Several studies have shown that the antiviral agent amantadine is a useful adjunct to vaccination for preventing influenza A in institutional settings. Currently, the proper use of inactivated vaccine according to the recommendations of public health authorities is the only way to reduce the annual influenza-associated medical and economic burden.