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

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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.