Influenza, commonly known as the flu, is a contagious respiratory illness caused by influenza viruses. Infection with influenza viruses can result in illness ranging from mild to severe and life-threatening complications.

There are three types of the virus:

All type A influenza viruses, including those that regularly cause seasonal epidemics of influenza in humans, are well adapted to elude host defenses. Influenza viruses lack mechanisms for the “proofreading” and repair of errors that occur during replication. As a result of these uncorrected errors, the genetic composition of the viruses changes as they replicate in humans and animals, and the existing strain is replaced with a new antigenic variant. These constant, permanent and usually small changes in the antigenic composition of influenza A viruses are known as antigenic “drift”.

The tendency of influenza viruses to undergo frequent and permanent antigenic changes necessitates constant monitoring of the global influenza situation and annual adjustments in the composition of influenza vaccines. Both activities have been a cornerstone of the WHO Global Influenza Program since its inception in 1947.

Influenza viruses have a second characteristic of great concern to public health: influenza A viruses, including subtypes from different species, can swap or “re-assort” genetic materials and merge. This re-assortment process, known as antigenic “shift”, results in a novel subtype different from both parent viruses. As populations will have no immunity to the new subtype, and as no existing vaccines can confer protection, antigenic shift has historically resulted in highly lethal pandemics. For antigenic shift to take place, the novel subtype needs to have genes from human influenza viruses that make it readily transmissible from person to person for a sustainable period.

Research has shown that antiviral drugs are effective for both the prevention (chemoprophylaxis) and early treatment of influenza, if administered within 48 hours following the onset of illness. During normal seasonal epidemics, antivirals are considered an important adjunct to vaccination as a strategy for reducing the medical and economic burden of influenza. Their use can reduce the duration of uncomplicated disease and the likelihood of complications requiring anti-microbial treatment and possibly hospitalization