At times, it would appear that the only constant feature of influenza is inconstancy. The antigenic structure of the influenza A virus undergoes frequent alterations. In some years, the alterations are relatively small (antigenic drift), while in others, they are drastic (antigenic shift). Antigenic shift or significant antigenic drift decreases the efficacy of a particular influenza vaccine, requiring frequent updating of the vaccine formulation. Published estimates of vaccine efficacy have varied widely since the vaccine's introduction in the early 1940s. In some studies, such as that by Foy et al in this issue of The Journal (p 1736), the incidence of respiratory illness did not differ noticeably between vaccinated and unvaccinated groups, while in other studies, the vaccine efficacy (ie, the percent of decrease in illness rates in vaccinees) has been as high as 90%.1
The practice of expressing vaccine efficacy as a single number belies the fact that
Nolan TF. Influenza Vaccine Efficacy. JAMA. 1981;245(17):1762. doi:10.1001/jama.1981.03310420052032
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