Surveillance systems require accurate indicators that detect possible epidemics in advance. The epidemic of influenza is one of the problems of most concern to public health professionals across the world, due to its high levels of mortality and morbidity. Influenza is highly contagious and causes more morbidity than any other vaccine-preventable illness . So, accurate estimates of the incidence of influenza are essential, for both public health services and citizens, to provide advance warning of epidemics and allow preventive measures to reduce contagion.