Influenza A or B viruses cause outbreaks of varying severity almost every winter, during which illness rates are commonly 10 to 15 percent in adults and higher in children. In the United States, influenza causes an average of over 20,000 deaths and over 200,000 hospitalizations during each epidemic. The appearance of a new strain for which most of the population lacks immunity can cause worldwide outbreaks, or pandemics. Pandemic strains are associated with global spread over months and with high attack rates in persons in all susceptible age groups. Five such pandemics have occurred during the past 100 years, the most severe of which was the swine or Spanish pandemic in 1918 and 1919, which caused over 20 million deaths worldwide.
Future pandemics are very likely.
The segmented nature of genome accounts for reassortment of RNA segments between two influenza viruses during dual infection and facilitates antigenic variation. Surface glycoprotein spikes process either hemagglutinin or neuraminidase activity. Hemagglutinin mediates early events in replication, specifically cell attachment and fusion of virus and cell membranes. By cleaving terminal sialic acid residues, neuraminidase promotes release of virus from infected cells and possibly penetration through respiratory secretions. Antibodies to neuraminidase inhibit release of virus from cells and correlate with reduced severity of illness rather than preventing infection.