Individual and community impact of influenza.

Influenza has been recognised as a distinct entity for centuries because of its profound effect on respiratory morbidity and mortality, although the full burden of the disease has been difficult to determine. The population groups most affected by influenza morbidity and mortality have also been recognised for years and are strongly related to the viral type or subtype circulating. Studies in the US suggest that the traditional pneumonia and influenza (P&I) mortality rate underestimates the total impact of influenza on mortality by a factor of approximately 3.8. Observations in different parts of the world confirm that mortality associated with P&I is not limited to specific countries. Vaccination not only reduces the severity of illness and death from P&I, but also reduces all-cause mortality, thus confirming these observations. Estimates of overall morbidity vary widely, depending on the methods used in various observational studies. The actual impact of influenza is likely to be within this range of estimates, but recent vaccination studies suggest the possibility of much higher rates of morbidity than previously estimated.