During 1998, severe outbreaks of influenza were observed in four swine herds in the United States. This event was unique because the causative agents, H3N2 influenza viruses, are infrequently isolated from swine in North America. In the second half of 2011, a number of U.S. residents were found to be infected with influenza A variant viruses, primarily H3N2. Investigations revealed human infections with these viruses following contact with swine as well as limited human-to-human transmission.
H3N2 is a subtype of the influenza virus A. Influenza A H3N2 Viruses can infect birds and mammals. In birds, humans, and pigs, the virus has mutated into many strains. By reassortment, influenza A H3N2 exchanges genes for internal proteins with other influenza subtypes. It is an important cause of human influenza. Influenza A H3N2 variant virus is an influenza virus that contains genes from human, avian and swine origins. Most infections with this virus have resulted in self-limited, mild respiratory illnesses. Influenza A H3N2 virus is very contagious and can cause severe illness especially in patients who are very young or old or have some other medical condition as well. The severity of symptoms can vary but usually involves respiratory and constitutional (e.g. headache, aching muscles) symptoms.
Influenza A H3N2 is increasingly abundant in seasonal influenza, which kills an estimated 36,000 people in the United States each year. Each seasonal Influenza A H3N2 flu is slightly different from one of last year's H3N2 variants. Seasonal influenza viruses flow out of overlapping epidemics in East and Southeast Asia, then trickle around the globe before dying off.
While influenza A H3N2 viruses have been detected in U.S. swine, it's unknown how widespread they are in swine herds. It's possible that sporadic infections and even localized outbreaks among people with H3N2 will continue to occur. While there is no evidence that sustained human-to-human transmission is occurring, all influenza viruses have the capacity to change and it's possible that this virus may become widespread. So far, the severity of illnesses associated with H3N2 in people has been similar to the severity of illnesses associated with seasonal flu virus infections. Limited serologic studies indicate that adults may have some pre-existing immunity to this virus while children do not.
Flu vaccines are based on predicting which mutants of H1N1, H3N2, H1N2, and influenza B 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 and rimantadine in H3N2 has increased from 1% in 1994 to 12% in 2003 to 91% in 2005. An A/Perth/16/2009-like H3N2 virus is included in the 2010-11 seasonal influenza vaccine. Perth-like H3N2 viruses were first identified in early 2009, but have not yet circulated widely in the United States. Past influenza vaccines did not contain this strain, so vaccination with last year's seasonal vaccine would not be expected to provide substantial protection against this H3N2 Perth-like strain.