Information on H3N2 Variant Influenza A Viruses

H3N2v Background

Since August 2011, a number of U.S. residents were found to be infected with influenza A variant viruses, primarily H3N2v. Investigations revealed human infections with these viruses following contact with swine as well as limited human-to-human transmission.

While H3N2v 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 this virus 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 this virus 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. CDC is closely monitoring human infections with all novel influenza viruses, including H3N2v viruses, and will provide more information as it becomes available.

For more general information about variant influenza viruses, see Variant (Swine Origin) Influenza Viruses in Humans.

For more detailed information about previous cases of H3N2v infection, see Reports of Human Infections with Variant Viruses.

Table. Case Count: Detected U.S. Human Infections with H3N2v by State since August 2011

This chart indicates the number of CDC-reported infections with variant influenza A viruses since August 2011. The following states have reported cases: Indiana (6), Iowa (3), Maine (2), Pennsylvania (3), Utah (1), and West Virginia (2).

Protect Yourself Against H3N2v

• Fact Sheet

Information for Health Professionals

Below is guidance for health care professional relevant to H3N2v.

• Prevention Strategies for Seasonal and Influenza A(H3N2)v in Health Care Settings
• Interim Guidance for Influenza Surveillance: Additional Specimen Collection for Detection of Influenza A(H3N2)v Virus Infections
• Interim Guidance on Specimen Collection, Processing, and Testing for Patients with Suspect Influenza A(H3N2)v Virus Infection
• Interim Guidance on Case Definitions to be Used For Investigations of Influenza A(H3N2)v Virus Cases

Information for Schools & Childcare Providers

• Guidance for School Administrators to Help Reduce the Spread of Seasonal Influenza in K-12 Schools

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