Increase in Influenza A H3N2v Virus Infections in Three U.S. States
Multiple infections with variant* influenza A (H3N2v) viruses have been identified in 3 states in recent weeks. From July 12 through August 3, 2012, 16 cases of H3N2v were reported and confirmed by CDC. This virus was first detected in humans in July 2011. It has also been isolated in U.S. swine in many U.S. states.
Since July 12, 2011, there have been 29 cases of H3N2v virus infection, including the 16 cases occurring in the last three weeks. All 29 cases were infected with H3N2v viruses that contain the matrix (M) gene from the influenza A (H1N1)pdm09 virus. This M gene may confer increased transmissibility to and among humans, compared to other variant influenza viruses. All cases have been laboratory-confirmed at CDC. Each of the 16 cases identified since July 12, 2012, reported contact with swine prior to illness onset; in 15 cases, contact occurred while attending or exhibiting swine at an agricultural fair.
While the viruses identified in these cases are genetically nearly identical, separate swine exposure events in each state were associated with human infections. There is no indication that the cases in different states are epidemiologically related.
Clinical characteristics of the 16 H3N2v recent cases have been generally consistent with signs and symptoms of seasonal influenza, and have included fever, cough, pharyngitis, myalgia, and headache. No hospitalizations or deaths have occurred among the 16 confirmed cases since July 2012. Public health and agriculture officials are investigating the extent of disease among humans and swine, and additional cases are likely to be identified as the investigation continues.