Overview

Influenza is a viral infection often referred to as the flu. Influenza viruses are RNA viruses from the family Orthomyxoviridae, and are classified as either type A or type B, which contain specific viral nucleoproteins (NP). The viral genome consists of eight unique viral RNA segments that are associated with the NP in a ribonucleoprotein complex. The virus structure consists of a lipid envelope and a layer of matrix protein that surround the ribonucleoprotein complex. Two proteins, the viral hemagglutinin protein (HA) and neuraminidase (NA) protein, project outward from the lipid envelope. The HA protein is involved in entry of the virus into cells and is the primary antigen involved in the immune response to infection or vaccination.

Influenza A viruses are classified into subtypes based on combinations of different HA and NA proteins. Influenza type A strains that infect humans usually have one of three HA proteins (H1, H2, H3) and one of two NA proteins (N1, N2). H3N2 and H1N1 are the most common influenza virus subtypes that cause seasonal influenza. Influenza virus strains are also identified by the location where they were isolated, the isolate number and the year of isolation. For example, an H3N2 influenza type A virus isolated from the eighth individual from whom a clinical sample was obtained in Puerto Rico in 1934 is designated as A/PR/8/34 (H3N2). There are no subtypes of influenza B virus, but many different strains exist under two primary lineages of influenza B, namely the Victoria and Yamagata lineages.

Influenza A viruses infect many mammalian species and birds, including migratory birds that can spread strains of influenza across the globe. Certain avian influenza viruses such as the H5N1 and H7N9 viruses are highly pathogenic and are associated with a high mortality when they infect humans. Fortunately, these viruses, except in rare instances, cannot be transmitted from humans to other humans. The possibility that these viruses may mutate and acquire the ability to be efficiently transmitted among humans and cause a highly lethal pandemic, remains a serious concern among public health officials.

New virus strains are derived from two types of events. Co-infection of animals such as pigs with different influenza viruses can allow mixing of viral genomes and re-assortment of viral RNA segments thus creating new viruses. This phenomenon is called antigenic shift. In addition, influenza viruses rapidly mutate causing new viral antigens to appear. This is called antigen drift. These two phenomena cause new influenza viruses to circulate each season and can in some years lead to widespread infections across the world (pandemics) such as occurred during the 2009 novel H1N1 pandemic.