Protective efficacy of live influenza vaccines. Its assessment in field and experimental conditions.

Cost benefit estimates of immunization programs are critically dependent on an accurate and reliable assessment of the protection offered by the vaccines. Consequently, the selection of appropriate parameters of protection becomes an issue of major importance. The protective efficacy of live influenza virus vaccines has been established for several vaccinal strains used in routine immunization. This was established under conditions of natural and artificial exposure to both homologous and heterologous influenza viruses. Reduction in the incidence of specific disease--respiratory illness, confirmed by influenza virus isolation and/or four-fold increase in serum HAI titer, was found to be significant. These data will be discussed in relation to available epidemiological models that have been developed for the evaluation of disease modifying properties of immunoprophylactic agents.