Influenza A virus subtype H3N2

Seasonal influenza is an annually recurring outbreak of flu, which occurs during the cold half of the year in each hemisphere. Annually, about 3 to 5 million cases of severe illness and 290,000 to 650,000 deaths from seasonal flu occur worldwide. A/H3N2 is one of the prevalent subtypes of flu which contribute to seasonal flu outbreaks, and a strain of A/H3N2 is usually included in the biannual reformulation of the flu vaccine. ==Swine flu==

A 2007 study reported: "In swine, three influenza A virus subtypes (H1N1, 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 (PB2 and PA) lineages. Present vaccination 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, 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 reassortment) with H5N1, passing genes and mutating into a form which can pass easily among humans. H3N2 evolved from H2N2 by antigenic shift 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 amantadine and rimantadine in H3N2 in humans had increased to 91% by 2005. In August 2004, researchers in China found H5N1 in pigs. ==Significant outbreaks==

Hong Kong Flu (1968–1969) (magnified about 100,000 times) The Hong Kong Flu was a flu pandemic caused by a strain of H3N2 descended from H2N2 by antigenic shift, 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 neuraminidase with the 1957 Asian flu (H2N2). Accumulated antibodies to the neuraminidase or internal proteins may have resulted in much fewer casualties than most pandemics. 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. 2017–2018 The 2017–2018 flu season was particularly severe, and at the time, it was the worst flu season since the 2009–2010 flu season, at the start of the 2009 swine flu pandemic. According to the director of the Centers for Disease Control and Prevention's influenza branch in January 2018, it was the first flu season where "we've had the entire continental US" show the same "widespread" flu activity, excluding the District of Columbia and Hawaii. Twenty-six of those states were classified by the CDC as having "high" activity in January 2018. This flu season was dominated by the H3N2 subtype, which is known for being the most severe of the 4 main seasonal flu subtypes. 2025–2026 In June 2025, seven mutations were found in a new strain of H3N2 seasonal flu, belonging to the Subtype K. This variant emerged at the tail end of the 2025 flu season in the Southern Hemisphere, after the WHO had already selected the J.2 subclade to use for the H3N2 component, for the upcoming winter's flu vaccine. This led to an earlier than usual start to the flu season in the United Kingdom, Canada, and Japan, and unusually severe flu activity in those countries. This triggered fears that the new H3N2 Subtype K virus could cause the worst flu season in at least a decade, which would produce two severe back-to-back flu seasons. The new H3N2 Subtype K variant had an estimated basic reproduction number (R_0) around 1.4, slightly higher than the typical R0 of 1.2 for seasonal flu strains. ==See also==