First H3N2 Variant Virus Infection Reported For 2012

April 12, 2012 – The first human infection with an influenza A H3N2 variant (H3N2v) virus in 2012 has been reported in a child by the state of Utah. CDC has confirmed this virus is very similar to the 12 H3N2v viruses that infected 12 people in the United States in the latter half of 2011. The virus contains genes from human, swine and avian influenza viruses, and the M gene from the 2009 H1N1 virus. Swine exposure has been reported for this case. Animal and public health investigations are currently underway to determine the source of this infection and if there are additional human cases. The report will be officially reported in the April 13, 2012 FluView influenza surveillance report.

After seeking medical care for a fever in late March, the patient in Utah tested positive for influenza A and was treated with oseltamivir, which is one of two antiviral drugs known as “neuraminidase inhibitors” currently recommended by CDC for the treatment of influenza virus infections, including infections with variant influenza viruses. The child recovered at home.

The case in Utah was detected through routine surveillance, when the influenza-positive sample from the medical visit was forwarded to the state public health laboratory for further characterization. Testing at the state public health laboratory identified the sample as a possible variant influenza virus. Follow-up testing at CDC confirmed that this is an influenza A H3N2v virus.

When a virus that is known to circulate in swine but not in people is identified in a person, it is called a “variant influenza virus.” The case in Utah brings the number of human infections with H3N2v viruses detected in the United States to 21 since July 2009. Thirteen of those 21 H3N2v viruses have had the 2009 H1N1 M gene and have occurred since July 2011. Twelve of these 13 cases have occurred in children younger than 18 years old. All patients have recovered from their illness. The previously identified 12 human infections with viruses like this one occurred in Indiana (2), Pennsylvania (3), Maine (2), Iowa (3) and West Virginia (2) between July and November 2011. None of the 13 viruses studied have any genetic markers known to confer resistance to the neuraminidase inhibitors oseltamivir (Tamiflu®) or zanamivir (Relenza®).

About half of the 12 infections in 2011 with H3N2v viruses like this one involved the patients being exposed to swine before becoming ill. The prevalence of this virus in swine is unknown, but it has been detected in U.S. swine through the United States Department of Agriculture’s (USDA) swine influenza surveillance program. Limited human-to-human transmission with this virus is suspected to have occurred in Iowa and West Virginia in November 2011.

An article in the April 13, 2011 Morbidity and Mortality Report Weekly Report entitled “Antibodies Cross-Reactive to Influenza A (H3N2) Variant Virus and Impact of 2010-11 Seasonal Influenza Vaccination on Cross-Reactive Antibodies” looked at whether a seasonal flu shot might create antibodies that would be protective of infection with these H3N2v viruses. The study found that the seasonal flu shot would not be expected to protect against H3N2v in young children, and would only provide limited protection against H3N2v viruses in adults. The study results imply that if these H3N2v viruses were to begin to spread easily and widely in people, an H3N2v-specific influenza vaccine would be needed to provide the best protection against H3N2v viruses.

CDC produced an H3N2v vaccine candidate virus. A precautionary vaccine against H3N2v is in development and will likely be ready for clinical trials in the coming months.

The total number of variant influenza virus infections in humans reported in the United States since December 2005 now stands at 36. The remaining 15 infections were with other variant influenza viruses, including H1N2v and H1N1v.

Human infections with influenza viruses that commonly circulate in swine are still rare events, but the frequency with which they have been detected has increased recently. These cases should be carefully investigated jointly by public and animal health officials to determine the source of the infection and ensure that no additional human infections occur or that there is no ongoing human-to-human transmission.

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