Seasonal Influenza

The regular “seasonal” bouts of influenza infections are principally caused by the circulating strains in the population undergoing antigenic drift. The virus causing influenza infection is transmitted in droplets in the air from human-to-human and targets epithelial cells in the trachea and bronchi of the upper respiratory tract. Influenza virus may also be picked up from contaminated surfaces and passed to the mouth. Disease spreads very quickly especially in crowded circumstances through coughing and sneezing. The stability of the virus is favored by low relative humidity and low temperatures and, as a consequence, seasonal epidemics in temperate areas tend to appear in winter. Greater morbidity and mortality is observed with influenza A strains, with influenza B usually associated with lower attack rates and a milder disease. Occasionally, however, influenza B can cause epidemics of the same severity as type A viruses. Influenza B affects only humans and is primarily a childhood pathogen; they do not exhibit the same degree of antigenic variation as type A.

During seasonal epidemics influenza can spread around the world quickly and inflicts a significant economic burden in terms of hospital and other healthcare costs and lost productivity. The incubation time for influenza can vary from one to five days The typical uncomplicated influenza infection is characterised by a rapid onset of illness (headache, cough, chills) followed by fever, sore throat, significant myalgias, malaise and loss of appetite. Further symptoms may include rhinorrhea, substernal tightness and ocular symptoms. The most prominent sign of infection is the fever that is usually in the 38-40oC range. The fever tends to subside on the second or third day of infection, and is resolved by day six.