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 f ull burden of the disease has been difficult to determine. The population g roups most affected by influenza morbidity and mortality have also been rec ognised for years and are strongly related to the viral type or subtype cir culating. 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 wo rld confirm that mortality associated with P&I is not limited to specific c ountries. Vaccination not only reduces the severity of illness and death fr om P&I, but also reduces all-cause mortality, thus confirming these observa tions. Estimates of overall morbidity vary widely, depending on the methods used i n various observational studies. The actual impact of influenza is likely t o be within this range of estimates, but recent vaccination studies suggest the possibility of much higher rates of morbidity than previously estimate d.