On Aug. 4, 2023, the U.S. Centers for Disease Control and Prevention (CDC) reported the first two U.S. human infections with swine flu viruses in 2023. These human infections were caused by two different types of flu viruses that normally spread among pigs, and they occurred in two people who attended different agricultural fairs in Michigan and had exposure to pigs. The CDC recommends people take precautions to prevent the spread of swine influenza viruses to people and has guidance for people exhibiting pigs at fairs, people attending fairs, and fair organizers.

Every year, there are rare sporadic human infections with flu viruses that usually spread in pigs. When found in people, these are called “variant flu virus” infections and designated with the letter “v” after the subtype. Variant flu virus infections are usually associated with contact with pigs, often at agricultural fairs. While infections with swine influenza viruses usually cause mild illness, they are concerning because they can cause severe illness, especially in people at higher risk of serious flu complications, and because of their pandemic potential. These infections are fully investigated to be sure that such viruses are not spreading in an efficient and ongoing way in people and to limit further exposure of people to infected animals if infected animals are identified.

The Michigan Department of Health and Human Services first reported a presumptive positive influenza A(H3) variant virus infection on July 26 in a person who attended a fair that took place July 7-16. A respiratory specimen from that patient was sent to CDC and received on July 27. Diagnostic testing conducted at CDC was inconclusive, likely due to the respiratory specimen not containing enough virus. However, it is likely the patient was infected with an influenza A(H3)v virus given the presumptive positive test results from Michigan, as well as the epidemiologic findings of the local investigation.