[Usefulness and limitations of vaccination against influenza]

Influenza viruses have caused major epidemics of respiratory disease and a high mortality in patients with advanced age and in those with underlying health conditions. It has been estimated in the USA that, during a moderate epidemic of influenza, there is an excess of 170,000 hospitalizations and approximately 20,000 to 40,000 influenza-associated deaths per years. Costs may exceed 1 billion dollars per year. Influenza viruses have been difficult to control because of a large animal reservoir and of the frequent change of the virus antigenic structure by major (shift) and minor (drift) variations. Current killed trivalent vaccines are made of two influenza A subtypes plus influenza B virus. Because of the antigenic variation, vaccine composition needs to be changed almost every year in order to contain the strains most likely to be effective. There are two types of vaccines--"whole" and "split". The latter has been treated to reduce pyrogenic components and is particularly indicated in children. Vaccine efficacy varies according to how closely the immunizing strains resemble the circulating wild strain. It also depends on the viral strains which had appeared in the preceding years (herd immunity), on the number of the doses administered and on the interval between the vaccination and the natural challenge. The antibody response to vaccine is short-lived. It is greatest in normal adults and lowest in aged and in immunosuppressed patients. Vaccination reduces clinical infections by 70-80% in children and adults, but only by 30% in elderly patients living in institutions.(ABSTRACT TRUNCATED AT 250 WORDS)