A critical evaluation of influenza surveillance.

The present system of global surveillance of influenza worked effectively in identifying the importance and character of the Hong Kong variant and informing the world of new developments as they occurred. Many deficiencies still exist in the surveillance system that require attention if it is to serve effectively in guiding future control of influenza. More quantitative data are needed regarding incidence of clinical disease, age- and sex-specific attack rates, character and severity of complications, socio-economic factors influencing mortality and distribution and utilization of influenza vaccines.The most valuable index for global surveillance of influenza is excess mortality due to respiratory causes. A comparison of the mortality experience in England and Wales with that in the USA reveals that sharply defined epidemics of influenza A recurred in both countries at intervals of 2-3 years. Mortality rates in England and Wales were considerably greater than in the USA and excess mortality during influenza epidemics was markedly higher in the former country with a notable exception in 1968-69, when the pandemic due to the Hong Kong variant was much more severe in the USA and the mortality in England and Wales was only slightly increased. Mortality data from many other parts of the world could be made available and would materially enhance the effectiveness of the global influenza surveillance programme.