Assessment of the efficacy of influenza vaccines against natural challenge.

Assessment of influenza vaccines by serological conversion rates or by protection against artificial challenge in animals or man give only a preliminary and indirect guide to their expected efficiency in controlling the natural epidemic disease. Field trials are essential but demand careful advance planning and control before meaningful results can be obtained. This paper reviews critically the various attempts that have been made to establish an ideal trial procedure to overcome the many inherent difficulties in studying 'wild' influenza. Vaccines of appropriate antigenic content can prevent influenza in 75 percent or more of recipients for up to a year, and can decrease the severity and duration of the disease in those who do succumb. Attack rates can be reduced even in epidemics occurring within 8-14 days of vaccination. If a high proportion of a population is vaccinated, infection rates in the unvaccinated in the same environment may fall. The desirability of vaccinating (a) only high risk groups or (b) entire open population or (c) school children is discussed.