Influenza A virus subtype H3N2 (A/H3N2) is a subtype of

virus A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Since Dmitri Ivanovsk ...

es that causes

influenza Influenza, commonly known as "the flu", is an infectious disease caused by influenza viruses. Symptoms range from mild to severe and often include fever, runny nose, sore throat, muscle pain, headache, coughing, and fatigue. These symptom ...

(flu). H3N2 viruses can infect birds and mammals. In birds, humans, and pigs, the virus has mutated into many strains. In years in which H3N2 is the predominant strain, there are more hospitalizations.

Classification

H3N2 is a subtype of the viral

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

Influenzavirus A, which is an important cause of human influenza. Its name derives from the forms of the two kinds of

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...

s on the surface of its coat,

hemagglutinin In molecular biology, hemagglutinins (or ''haemagglutinin'' in British English) (from the Greek , 'blood' + Latin , 'glue') are receptor-binding membrane fusion glycoproteins produced by viruses in the '' Paramyxoviridae'' family. Hemagglutinins a ...

(H) and

neuraminidase Exo-α-sialidase (EC 3.2.1.18, sialidase, neuraminidase; systematic name acetylneuraminyl hydrolase) is a glycoside hydrolase that cleaves the glycosidic linkages of neuraminic acids: : Hydrolysis of α-(2→3)-, α-(2→6)-, α-(2→8)- glyc ...

(N). By

reassortment Reassortment is the mixing of the genetic material of a species into new combinations in different individuals. Several different processes contribute to reassortment, including assortment of chromosomes, and chromosomal crossover. It is particul ...

, H3N2 exchanges genes for internal proteins with other influenza subtypes.

Seasonal H3N2 flu

Seasonal influenza Flu season is an annually recurring time period characterized by the prevalence of an outbreak of influenza (flu). The season occurs during the cold half of the year in each hemispheres of Earth, hemisphere. It takes approximately two days to ...

kills an estimated 36,000 people in the United States each year. Flu

vaccines A vaccine is a biological preparation that provides active acquired immunity to a particular infectious or malignant disease. The safety and effectiveness of vaccines has been widely studied and verified.

are based on predicting which "mutants" of

H1N1 In virology, influenza A virus subtype H1N1 (A/H1N1) is a subtype of influenza A virus. Major outbreaks of H1N1 strains in humans include the Spanish flu, the 1977 Russian flu pandemic and the 2009 swine flu pandemic. It is an orthomyxoviru ...

, H3N2, H1N2, and

influenza B ''Influenza B virus'' is the only species in the genus ''Betainfluenzavirus'' in the virus family ''Orthomyxoviridae''. Influenza B virus is known only to infect humans and seals. This limited host range is apparently responsible for the lack ...

will proliferate in the next season. Separate vaccines are developed for the Northern and Southern Hemispheres in preparation for their annual epidemics. In the tropics, influenza shows no clear seasonality. In the past ten years, H3N2 has tended to dominate in prevalence over H1N1, H1N2, and influenza B. Measured resistance to the standard antiviral drugs

amantadine Amantadine, sold under the brand name Gocovri among others, is a medication used to treat dyskinesia associated with parkinsonism and influenza caused by type A influenzavirus, though its use for the latter is no longer recommended due to wid ...

and rimantadine in H3N2 has increased from 1% in 1994 to 12% in 2003 to 91% in 2005. Seasonal H3N2 flu is a human flu from H3N2 that is slightly different from one of the previous year's flu season H3N2 variants. Seasonal influenza viruses flow out of overlapping epidemics in

East Asia East Asia is the eastern region of Asia, which is defined in both geographical and ethno-cultural terms. The modern states of East Asia include China, Japan, Mongolia, North Korea, South Korea, and Taiwan. China, North Korea, South Korea ...

and

Southeast Asia Southeast Asia, also spelled South East Asia and South-East Asia, and also known as Southeastern Asia, South-eastern Asia or SEA, is the geographical south-eastern region of Asia, consisting of the regions that are situated south of mainland ...

, then trickle around the globe before dying off. Identifying the source of the viruses allows global health officials to better predict which viruses are most likely to cause the most disease over the next year. An analysis of 13,000 samples of influenza A/H3N2 virus that were collected across six continents from 2002 to 2007 by the WHO's

Global Influenza Surveillance Network National Influenza Centers (also called National Influenza Centres) are institutions which are formally recognized as such by the World Health Organization (WHO).Australia Australia, officially the Commonwealth of Australia, is a sovereign country comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands. With an area of , Australia is the largest country by ...

and

New Zealand New Zealand ( mi, Aotearoa ) is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and over 700 smaller islands. It is the sixth-largest island coun ...

next, followed by

North America North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and th ...

and Europe. The new variants typically reached South America after an additional six to nine months, the group reported.

Swine flu

A 2007 study reported: "In swine, three influenza A virus subtypes (

, H3N2, and H1N2) are circulating throughout the world. In the United States, the classic H1N1 subtype was exclusively prevalent among swine populations before 1998; however, since late August 1998, H3N2 subtypes have been isolated from pigs. Most H3N2 virus isolates are triple reassortants, containing genes from human (HA, NA, and PB1), swine (NS, NP, and M), and

avian Avian may refer to: * Birds or Aves, winged animals *Avian (given name) (russian: Авиа́н, link=no), a male forename Aviation *Avro Avian, a series of light aircraft made by Avro in the 1920s and 1930s *Avian Limited, a hang glider manufactur ...

(PB2 and PA) lineages. Present

vaccination Vaccination is the administration of a vaccine to help the immune system develop immunity from a disease. Vaccines contain a microorganism or virus in a weakened, live or killed state, or proteins or toxins from the organism. In stimulat ...

strategies for swine influenza virus (SIV) control and prevention in swine farms typically include the use of one of several bivalent SIV vaccines commercially available in the United States. Of the 97 recent H3N2 isolates examined, only 41 had strong serologic cross-reactions with antiserum to three commercial SIV vaccines. Since the protective ability of influenza vaccines depends primarily on the closeness of the match between the vaccine virus and the epidemic virus, the presence of nonreactive H3N2 SIV variants suggests current commercial vaccines might not effectively protect pigs from infection with a majority of H3N2 viruses." Avian influenza virus H3N2 is endemic in pigs in

China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's List of countries and dependencies by population, most populous country, with a Population of China, population exceeding 1.4 billion, slig ...

, and has been detected in pigs in Vietnam, contributing to the emergence of new variant strains. Pigs can carry human influenza viruses, which can combine (i.e. exchange homologous genome subunits by genetic

) with

H5N1 Influenza A virus subtype H5N1 (A/H5N1) is a subtype of the influenza A virus which can cause illness in humans and many other animal species. A bird-adapted strain of H5N1, called HPAI A(H5N1) for highly pathogenic avian influenza virus of type ...

, passing

gene In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...

s and mutating into a form which can pass easily among humans. H3N2 evolved from

H2N2 Influenza A virus subtype H2N2 (A/H2N2) is a subtype of ''Influenza A virus''. H2N2 has mutated into various strains including the "Asian flu" strain (now extinct in the wild), H3N2, and various strains found in birds. It is also suspected of ...

antigenic shift Antigenic shift is the process by which two or more different strains of a virus, or strains of two or more different viruses, combine to form a new subtype having a mixture of the surface antigens of the two or more original strains. The term is ...

and caused the Hong Kong Flu pandemic of 1968 and 1969 that killed up to 750,000 humans. The dominant strain of annual flu in humans in January 2006 was H3N2. Measured resistance to the standard antiviral drugs

and rimantadine in H3N2 in humans had increased to 91% by 2005. In August 2004, researchers in China found H5N1 in pigs.

Flu spread, by season

Hong Kong Flu (1968–1969)

The Hong Kong Flu was a

flu pandemic An influenza pandemic is an epidemic of an influenza virus that spreads across a large region (either multiple continents or worldwide) and infects a large proportion of the population. There have been six major influenza epidemics in the las ...

caused by a strain of H3N2 descended from

, in which genes from multiple subtypes reassorted to form a new virus. This pandemic of 1968 and 1969 killed an estimated one million people worldwide. The pandemic infected an estimated 500,000 Hong Kong residents, 15% of the population, with a low death rate. In the United States, about 100,000 people died. Both the H2N2 and H3N2 pandemic flu strains contained genes from avian influenza viruses. The new subtypes arose in pigs coinfected with avian and human viruses and were soon transferred to humans. Swine were considered the original "intermediate host" for influenza, because they supported reassortment of divergent subtypes. However, other hosts appear capable of similar coinfection (e.g., many poultry species), and direct transmission of avian viruses to humans is possible. H1N1 may have been transmitted directly from birds to humans (Belshe 2005). The Hong Kong flu strain shared internal genes and the

with the 1957

Asian flu Asian may refer to: * Items from or related to the continent of Asia: ** Asian people, people in or descending from Asia ** Asian culture, the culture of the people from Asia ** Asian cuisine, food based on the style of food of the people from Asi ...

(H2N2). Accumulated antibodies to the neuraminidase or internal proteins may have resulted in much fewer casualties than most

pandemic A pandemic () is an epidemic of an infectious disease that has spread across a large region, for instance multiple continents or worldwide, affecting a substantial number of individuals. A widespread endemic disease with a stable number of in ...

s. However, cross-immunity within and between subtypes of influenza is poorly understood. The Hong Kong flu was the first known outbreak of the H3N2 strain, though there is serologic evidence of H3N2 infections in the late 19th century. The first record of the outbreak in Hong Kong appeared on 13 July 1968 in an area with a density of about 500 people per acre in an urban setting. The outbreak reached maximum intensity in two weeks, lasting six weeks in total. The virus was isolated in Queen Mary Hospital. Flu symptoms lasted four to five days. By July 1968, extensive outbreaks were reported in Vietnam and

Singapore Singapore (), officially the Republic of Singapore, is a sovereign island country and city-state in maritime Southeast Asia. It lies about one degree of latitude () north of the equator, off the southern tip of the Malay Peninsula, bor ...

. By September 1968, it reached India, the Philippines, northern Australia and Europe. That same month, the virus entered

California California is a state in the Western United States, located along the Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the most populous U.S. state and the 3rd largest by area. It is also the m ...

from United States troops returning from the

Vietnam War The Vietnam War (also known by #Names, other names) was a conflict in Vietnam, Laos, and Cambodia from 1 November 1955 to the fall of Saigon on 30 April 1975. It was the second of the Indochina Wars and was officially fought between North Vie ...

. It reached Japan, Africa and South America in 1969.

Fujian flu (2003–2004)

Fujian flu refers to flu caused by either a Fujian human flu strain of the H3N2 subtype or a Fujian bird flu strain of the H5N1 subtype of the Influenza A virus. These strains are named after

Fujian Fujian (; alternately romanized as Fukien or Hokkien) is a province on the southeastern coast of China. Fujian is bordered by Zhejiang to the north, Jiangxi to the west, Guangdong to the south, and the Taiwan Strait to the east. Its ...

province in China. A/Fujian (H3N2) human flu (from A/Fujian/411/2002(H3N2)-like flu virus strains) caused an unusually severe 2003–2004 flu season. This was due to a

event that caused a minor clade to provide a

haemagglutinin In molecular biology, hemagglutinins (or ''haemagglutinin'' in British English) (from the Greek , 'blood' + Latin , 'glue') are receptor-binding membrane fusion glycoproteins produced by viruses in the ''Paramyxoviridae'' family. Hemagglutinins ar ...

gene that later became part of the dominant strain in the 2002–2003 flu season. A/Fujian (H3N2) was made part of the trivalent influenza vaccine for the 2004–2005 flu season.

2004–2005 flu season

The 2004–05 trivalent influenza vaccine for the United States contained: * an A/New Caledonia/20/99 (H1N1)-like virus * an A/Fujian/411/2002 (H3N2)-like virus * a B/Shanghai/361/2002-like virus.CDC
article ''Update: Influenza Activity—United States and Worldwide, 2003–04 Season, and Composition of the 2004–05 Influenza Vaccine'' published 2 July 2004

2005–2006 flu season

The

vaccine A vaccine is a biological preparation that provides active acquired immunity to a particular infectious or malignant disease. The safety and effectiveness of vaccines has been widely studied and verified.

s produced for the 2005–2006 season used: * an A/New Caledonia/20/1999-like(

) * an A/California/7/2004-like( H3N2) (or the antigenically equivalent strain A/New York/55/2004) * a B/Jiangsu/10/2003-like viruses

2006–2007 flu season

The 2006–2007 influenza vaccine composition recommended by the World Health Organization on 15 February 2006 and the US FDA's Vaccines and Related Biological Products Advisory Committee on 17 February 2006 used: * an A/New Caledonia/20/99 (H1N1)-like virus * an A/Wisconsin/67/2005 (H3N2)-like virus (A/Wisconsin/67/2005 and A/Hiroshima/52/2005 strains) * a B/Malaysia/2506/2004-like virus from B/Malaysia/2506/2004 and B/Ohio/1/2005 strains which are of B/Victoria/2/87 lineage

2007–2008 flu season

The composition of influenza virus vaccines for use in the 2007–2008 Northern Hemisphere influenza season recommended by the World Health Organization on 14 February 2007 was: * an A/Solomon Islands/3/2006 (H1N1)-like virus * an A/Wisconsin/67/2005 (H3N2)-like virus (A/Wisconsin/67/2005 (H3N2) and A/Hiroshima/52/2005 were used at the time) * a B/Malaysia/2506/2004-like virus "A/H3N2 has become the predominant flu subtype in the United States, and the record over the past 25 years shows that seasons dominated by H3N2 tend to be worse than those dominated by type A/H1N1 or type B." Many H3N2 viruses making people ill in this 2007–2008 flu season differ from the strains in the vaccine and may not be well covered by the vaccine strains. "The CDC has analyzed 250 viruses this season to determine how well they match up with the vaccine, the report says. Of 65 H3N2 isolates, 53 (81%) were characterized as A/Brisbane/10/2007-like, a variant that has evolved otablyfrom the H3N2 strain in the vaccine—A/Wisconsin/67/2005."

2008–2009 flu season

The composition of virus vaccines for use in the 2008–2009 Northern Hemisphere influenza season recommended by the World Health Organization on February 14, 2008 was: * an A/Brisbane/59/2007 (H1N1)-like virus * an A/Brisbane/10/2007 (H3N2)-like virus * a B/Florida/4/2006-like virus (B/Florida/4/2006 and B/Brisbane/3/2007 (a B/Florida/4/2006-like virus) were used at the time) As of May 30, 2009: "CDC has antigenically characterized 1,567 seasonal human influenza viruses 47 influenza A (H1), 162 influenza A (H3) and 458 influenza B virusescollected by U.S. laboratories since October 1, 2008, and 84 novel influenza A (H1N1) viruses. All 947 influenza seasonal A (H1) viruses are related to the influenza A (H1N1) component of the 2008–09 influenza vaccine (A/Brisbane/59/2007). All 162 influenza A (H3N2) viruses are related to the A (H3N2) vaccine component (A/Brisbane/10/2007). All 84 novel influenza A (H1N1) viruses are related to the A/California/07/2009 (H1N1) reference virus selected by WHO as a potential candidate for novel influenza A (H1N1) vaccine. Influenza B viruses currently circulating can be divided into two distinct lineages represented by the B/Yamagata/16/88 and B/Victoria/02/87 viruses. Sixty-one influenza B viruses tested belong to the B/Yamagata lineage and are related to the vaccine strain (B/Florida/04/2006). The remaining 397 viruses belong to the B/Victoria lineage and are not related to the vaccine strain."CDC
article "2008–2009 Influenza Season Week 21 ending May 30, 2009" published May 30, 2009

2009–2010 flu season

The

s produced for the 2009–2010 season used: * an A/Brisbane/59/2007(H1N1)-like virus * an A/Brisbane/10/2007 (H3N2)-like virus * a B/Brisbane 60/2008-like antigens A separate vaccine was available for pandemic H1N1 influenza using the A/California/7/2009-like pandemic H1N1 strain.

2010–2011 flu season

The

s produced for the 2010–2011 season used: * an A/California/7/2009-like (pandemic H1N1) * an A/Perth/16/2009-like (H3N2)-like virus * a B/Brisbane/60/2008-like antigens

2011–2012 flu season

The

s produced for the 2011–2012 season used: * an A/California/07/2009 (H1N1)-like virus * an A/Victoria/210/2009 (an A/Perth/16/2009-like strain) (H3N2)-like virus * a B/Brisbane/60/2008-like virus

2012–2013 flu season

The

s produced for the Northern Hemisphere 2012–2013 season used: * an A/California/07/2009 (H1N1)-like virus * an A/Victoria/361/2011 (H3N2)-like virus * a B/Massachusetts/2/2012-like virus, which replaced B/Wisconsin/1/2010-like virus In January 2013, influenza activity continued to increase in the United States and most of the country experienced high levels of influenza-like-illness (ILI), according to CDC's latest FluView report. Reports of influenza-like-illness (ILI) are nearing what have been peak levels during moderately severe seasons, and CDC continues to recommend influenza vaccination and

antiviral drug Antiviral drugs are a class of medication used for treating viral infections. Most antivirals target specific viruses, while a broad-spectrum antiviral is effective against a wide range of viruses. Unlike most antibiotics, antiviral drugs do n ...

treatment when appropriate at this time. On January 9, 2013, the Boston Government declared a public health emergency for H3N2 influenza.

2014–2015 flu season

The

s produced for the Northern Hemisphere 2014–2015 season used: * A/California/7/2009 (H1N1)pdm09-like virus * A/Texas/50/2012 (H3N2)-like virus * B/Massachusetts/2/2012-like virus, Quadrivalent vaccines include a B/Brisbane/60/2008-like virus. The CDC announced that drift variants of the A (H3N2) virus strain from the 2012–2013 potentially foretold a severe flu season for 2014–2015.

2015–2016 flu season

The

s produced for the Northern Hemisphere 2015–2016 season used: * A/California/7/2009 (H1N1)pdm09-like virus * A/Switzerland/9715293/2013 (H3N2)-like virus * B/Phuket/3073/2013-like virus. (This is a B/Yamagata lineage virus) The "Split Virion" vaccine distributed in 2016 contained the following strains of inactivated virus: * A/California/7/2009 (H1N1)pdm09 - like strain (A/California/7/2009, NYMC X-179A) * A/Hong Kong/4801/2014 (H3N2) - like strain (A/Hong Kong/4801/2014, NYMC X-263B) * B/Brisbane/60/2008 - like strain (B/Brisbane/60/2008, wild type)

2016–2017 flu season

* A/California/7/2009 (H1N1)pdm09-like virus, * A/Hong Kong/4801/2014 (H3N2)-like virus * B/Brisbane/60/2008-like virus (B/Victoria lineage) Quadrivalent influenza vaccine adds: * B/Phuket/3073/2013-like strain

2017–2018 flu season

* A/Michigan/45/2015 (H1N1)pdm09-like virus * A/Hong Kong/4801/2014 (H3N2) * B/Brisbane/60/2008-like virus (B/Victoria lineage) Quadrivalent influenza vaccine adds * B/Phuket/3073/2013-like /Yamagata lineage

2020-SH(southern hemisphere)

Quadrivalent- A/Brisbane/02/2018 (H1N1)pdm09-like virus A/SouthAustralia/34/2019 (H3N2)-like virus B/Washington/02/2019-like virus /Victoria lineage B/Phuket/3073/2013-like virus /Yamagata lineage

References

External links

Influenza Research Database
Database of influenza sequences and related information. {{DEFAULTSORT:Influenza A Virus Subtype H3n2

H3N2 Influenza A virus subtype H3N2 (A/H3N2) is a subtype of viruses that causes influenza (flu). H3N2 viruses can infect birds and mammals. In birds, humans, and pigs, the virus has mutated into many strains. In years in which H3N2 is the predomina ...