Influenza A (H3N2)v: What Goes Around Comes Around

The original article about how influenza A (H3N2)v may have changed to become more infective in humans was first published in January 2012. However, since that time, the situation with the virus has changed again. This article update will present the newest findings on H3N2v and some insights made by the CDC. To gain a perspective about how fast situations with influenza A viruses change, the short original article will be left essentially intact and the new information will appear starting in paragraph seven.

In the early 1990s, a human strain of flu virus, influenza A H3N2, was documented to infect pigs. It's relatively rare to document this transfer of infective flu virus from humans to pigs, but it can happen when surveillance tests of human and animal infections detect the viruses and identify them. This same type of flu virus transfer can occur from pigs to humans; this occurred in the 2009 "swine flu" pandemic with the influenza A H1N1 strain of virus. Usually there is some major or at least minor change or modification of the flu virus genome that allows the flu virus to more easily adapt to infecting, replicating, and reinfecting a new host population. This flu season that officially began in October 2011 has a new flu type detected in it; the virus is closely related to that influenza A H3N2 virus that infected pigs in the 1990s, and this time the evidence suggests the infected pigs transferred the virus back to humans with some modifications. The modified virus is termed influenza A (H3N2)v.

The CDC reported that a reassortment of genetic material from H1N1 and H3N2 viruses resulted in a strain termed influenza A (H3N2)v, where an M gene from H1N1 virus was detected in the H3N2 viruses. New terminology rules, agreed to by the CDC, WHO, and other agencies, resulted in swine-origin influenza viruses found in humans to be called "variant" viruses and should be designated with a "v" after their name. Consequently, the new virus is termed influenza A (H3N2)v.

There have been only 12 confirmed patients with (H3N2)v; six were confirmed to be infected from direct contact with pigs infected with the virus while six others had no recent exposure to any pigs. Because this strain has been detected in at least five states to date, there is some concern it may become widespread. Although this year's trivalent vaccine against the flu contains an H3N2 strain antigen, it is not close enough antigenically to (H3N2)v to provide significant protection. However, people who are older and were infected in the 1990s with the H3N2 virus may have some residual protective immunity to (H3N2)v. The most susceptible human populations to the new virus, according to the CDC, are children, pregnant women, people 65 years of age or older, and patients with chronic diseases (for example, diabetes, asthma, or heart disease). The CDC has reported no deaths from (H3N2)v and suggested that patients be treated similarly to any other flu patients. They also state that the viruses are currently susceptible to oseltamivir (Tamiflu) and zanamivir (Relenza).

With only 12 people documented (by rRT-PCR tests confirmed by CDC labs) to have flu caused by (H3N2)v, why the concern? First, there is concern because the USDA Agricultural Research Service have detected at least eight isolates of (H3N2)v in pig populations, suggesting there may be an increasing source for infection of humans. Secondly, about half of the detected (H3N2)v infections were in children without any recent exposure to pigs, suggesting that person-to-person transfer of (H3N2)v can occur. Finally, of all infected people, 11 of 12 were children under the age of 18. The concern is that as the flu season progresses, it may be possible that many more people, especially susceptible children, may become infected, and there is, like in the start of the H1N1 pandemic, no effective vaccine currently available.

This situation again shows how closely humans are to other species in terms of being susceptible to certain similar viral diseases. It should make us aware of the cyclic nature of certain diseases that can develop over time as populations in both animals and humans develop large numbers of either resistant or susceptible subpopulations. Understanding how "what (diseases) go around will come around" may allow us to develop better testing methods, vaccines, and treatments to interrupt this cycle.


The above information was published in January; since then, the situation with the H3N2v virus has changed…again. In July 2012, the CDC noted a significant increase in the number of humans with H3N2v infections. The majority of patients were children or workers at state fairs or in the swine-raising industry, so most of the patients had an association with pigs. Studies have shown that many pig populations in several U.S. states are infected with the H3N2v virus. As of August 2012, to date, about 225 human infections with H3N2v have been reported (Indiana and Ohio have the highest number of H3N2v infections). The CDC suggests that the infection seems to be spreading from pigs to humans and has detected no easy transfer of the virus between humans. They do caution, however, that with the rapid genetic changes that can occur with influenza A viruses, human-to-human transmission may occur in the future, especially since the strain has acquired the M (matrix) gene from the 2009 H1N1 pandemic virus. Currently, there have been no deaths caused by this virus and only a few (eight) hospitalizations; the CDC currently thinks the clinical evidence suggests H3N2v produces illness no more severe than the seasonal flu. However, with the influenza A viruses like H3N2v, the CDC plans to keep a close watch on how the flu progresses this season.

The following are the new CDC recommendations to avoid getting H3N2v infections from pigs:

CDC Recommendations for People at High Risk

  • If you are at high risk of serious flu complications and are going to a fair where pigs will be present, avoid pigs and swine barns at the fair this year. This includes children younger than 5 years of age, people 65 years of age and older, pregnant women, and people with certain long-term health conditions (like asthma, diabetes, heart disease, weakened immune systems, and neurological or neurodevelopmental conditions).

CDC Recommendations for People Not at High Risk

  • Don't take food or drink -- or eat, drink, or put anything in your mouth -- in pig areas.
  • Don't take toys, pacifiers, cups, baby bottles, strollers, or similar items into pig areas.
  • Wash your hands often with soap and running water before and after exposure to pigs. If soap and water are not available, use an alcohol-based hand rub.
  • Avoid close contact with pigs that look or act ill.
  • Take protective measures if you must come in contact with pigs that are known or suspected to be sick. This includes wearing protective clothing, gloves, masks that cover your mouth and nose, and other personal protective equipment.
  • Watch your pig (if you have one) for signs of illness and call a veterinarian if you suspect it might be sick.
  • Avoid contact with pigs if you have flu-like symptoms. Wait seven days after your illness started or until you have been without fever for 24 hours without the use of fever-reducing medications, whichever is longer. If you must have contact with pigs while you are sick, take the protective actions listed above.


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The CDC also points out that current seasonal flu vaccine will not protect individuals from H3N2v infection but does mention that antiviral drugs (listed above in the article) may be helpful in reducing or eliminating symptoms. Influenza A H3N2v may change in the future so this article topic may come around...again.

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United States. Centers for Disease Control and Prevention. "Information on Influenza A (H3N2) Variant Viruses ("H3N2v")." Aug. 20, 2012. <>.

United States. Centers for Disease Control and Prevention. "More H3N2v Cases Reported, Still Linked to Pig Exposure." Aug. 17, 2012. <>.

United States. Centers for Disease Control and Prevention. "Seasonal Influenza (Flu)." Dec. 23, 2011. <>.

United States. Centers for Disease Control and Prevention. "Take Action to Prevent the Spread of Flu Between People and Pigs at Fairs." Aug. 20, 2012. <>.

United States. Centers for Disease Control and Prevention. "Update: Influenza A (H3N2)v Transmission and Guidelines - Five States, 2011." MMWR 60.51 Jan. 6, 2012: 1741-1744. <>.