How is influenza classified

There are two main types of human flu viruses: types A and B. The flu A and B viruses that routinely spread in people are responsible for seasonal flu epidemics each year. Flu A viruses can be broken down into sub-types depending on the genes that make up the surface proteins. To receive weekly email updates about Seasonal Flu, enter your email address:. Skip directly to site content Skip directly to page options Skip directly to A-Z link. Influenza Flu. In , a human-avian reassortant produced an antigenic shift to H2N2.

This virus was termed the Asian flu because it originated in China and then spread worldwide. It lasted until and caused between 1 and 4 million deaths. Since the antigenic shift involved only hemagglutinin, this pandemic was less extensive than that seen in , causing , deaths.

H3N2 influenza A reoccurred in late and early It is currently endemic in both human and pig populations. In , an antigenic shift produced an H1N1 influenza A virus that affected primarily young individuals who lacked preexisting immunity to H1N1. Since the late s, triple reassortant swine influenza A viruses containing genes from swine, human, and avian strains of influenza have been detected among swine herds in North America. In March , an outbreak of H1N1 influenza A virus was detected in Mexico that spread rapidly to the United States, Canada, and throughout the world.

The pandemic was declared to be over in August This pandemic was caused by a quadruple reassortment of two swine strains, one human strain, and one avian strain of influenza. The virus was called Swine Flu because the largest proportion of genes came from swine influenza viruses.

In response to the potential for a major pandemic, a mass vaccination campaign using an H1N1 monovalent vaccine was undertaken. Although the WHO declared this pandemic over in , this H1N1 strain continues to circulate around the globe along with the seasonal flu. H1N1 was included in the seasonal flu vaccine. Novel influenza A viruses that normally circulate in animals may infect humans. As of December 23, , the Centers for Disease Control and Prevention CDC has received reports of 35 cases of human infection with swine origin variant influenza viruses since The frequency with which these variant influenza viruses have been detected has increased since In the last 6 months of , 12 US residents in 5 different states Indiana, Iowa, Maine, Pennsylvania, and West Virginia were found to be infected with this influenza A H3N2 variant virus that had genes from avian, swine, and human influenza viruses.

This virus differs from the other cases because it has acquired another genetic change, the matrix M gene from the H1N1 pandemic virus. At this time, no one knows what the addition of this M gene means in terms of illness severity or transmissibility in humans. So far, most illness associated with this virus has been mild and self-limited. Limited serologic studies conducted recently at CDC indicate that adults may have some preexisting immunity to H3N2v, whereas children likely do not.

The viral RNAs that make up the genome are shown as red coils inside the particle and bound to ribonuclear proteins RNPs. Source: NIH, public domain. The nomenclature used to describe a specific influenza virus was established by the World Health Organization in and is expressed in this order:. This is translated as: Influenza type A, isolated first in California, lineage strain number 04, year , and type H1N1.

This is translated as: Influenza type A, first isolated in Fujian a province on the Southeast coast of mainland China , lineage number , year , type H3N2. Influenza virus nomenclature for a Fujian flu virus. Source: Wikipedia. The Fujian H3N2 influenza of caused an unusually severe — flu season, partly because it spread rapidly and partly because the vaccine for that season had already been formulated when the Fujian H3N2 virus was identified.

Type A influenza viruses are divided into subtypes , based on the presence of two glycoproteins on the surface of the virus. These glycoproteins are called hemagglutinin HA and neuraminidase NA. About 18 hemagglutinins have been identified, although generally, only H1, H2, and H3 are found in human influenza viruses.

There are more than types of neuraminidase, but only N1 and N2 have been positively linked to influenza epidemics in humans. The above image shows the features of an influenza virus, including the surface proteins hemagglutinin HA and neuraminidase NA. By binding to these antigenic sites, antibodies neutralize flu viruses and prevent them from causing further infection. Source: CDC. Hemagglutinin and neuraminidase are also called antigens , substances that, when introduced into the body, stimulate the production of an antibody.

Currently, there are two subtypes of influenza A viruses found circulating among human populations: influenza A H1N1 and influenza A H3N2. A reservoir is the place where a pathogen lives and survives. For all subtypes of influenza A viruses, wild birds are the primary natural reservoir and are thought to be the source of influenza A viruses in all other animals.

Influenza A viruses are found in many different animals, including ducks, chickens, pigs, whales, horses, and seals. Most influenza viruses cause asymptomatic or mild infection in birds; however, clinical signs in birds vary greatly depending on the virus.

Infection with certain avian influenza A viruses for example, some H5 and H7 viruses can cause widespread, severe disease and death among some species of birds CDC, a. Zhejiang province, China in red. Influenza A H3N2 viruses also change both genetically and antigenically. Influenza A H3N2 viruses have formed many separate, genetically different clades in recent years that continue to co-circulate.

Similar to influenza A viruses, influenza B viruses can then be further classified into specific clades and sub-clades. Influenza B viruses generally change more slowly in terms of their genetic and antigenic properties than influenza A viruses, especially influenza A H3N2 viruses.

Influenza surveillance data from recent years shows co-circulation of influenza B viruses from both lineages in the United States and around the world.

However, the proportion of influenza B viruses from each lineage that circulate can vary by geographic location and by season. Figure 2 — This image shows how influenza viruses are named. The name starts with the virus type, followed by the place the virus was isolated, followed by the virus strain number often a sample identifier , the year isolated, and finally, the virus subtype.

CDC follows an internationally accepted naming convention for influenza viruses. This convention was accepted by WHO in and published in February in the Bulletin of the World Health Organization, 58 4 see A revision of the system of nomenclature for influenza viruses: a WHO Memorandum pdf icon[ KB, 7 pages]external icon pdf icon external icon. The approach uses the following components:. Getting a flu vaccine can protect against these viruses as well as additional flu viruses that are antigenically similar to the viruses used to make the vaccine.

Seasonal flu vaccines do not protect against influenza C or D viruses or against zoonotic animal-origin flu viruses that can cause human infections, such as variant or avian flu viruses.

In addition, flu vaccines will NOT protect against infection and illness caused by other viruses that also can cause influenza-like symptoms.

There are many other viruses besides influenza that can result in influenza-like illness ILI that spread during flu season.