CDC Reports First U.S. Human Infection in 2024 with Variant Influenza Virus

CDC update

April 5, 2024 —On March 29, 2024, CDC reported this year's first U.S. human infection with an influenza (flu) virus that normally spreads in pigs and not people. The infection with an influenza A(H1N2) variant (v) virus occurred in a child living near a pig farm in Pennsylvania who had direct contact with pigs prior to illness onset. The person was hospitalized and has since recovered from their illness. CDC recommends that people in contact with pigs take precautions and provides specific guidance for people who are at higher risk of developing serious complications from flu.

The patient sought health care during the week ending March 9, 2024, and had the following symptoms: fever, vomiting, cough, and rhinorrhea (runny nose). The hospitalized patient received influenza antiviral treatment and recovered. The case was first reported by the Pennsylvania Department of Health. Local public health officials found that the patient had swine contact prior to illness onset. No person-to-person spread of the A(H1N2)v virus associated with this patient has been identified.

Additional investigation determined that two of the patient's close contacts had also contact with swine and had been ill before the patient developed symptoms. The two persons had mild illness; their symptoms resolved, and they were not tested for influenza.

Variant influenza virus infections usually cause mild illness; however, they can cause severe illness and are concerning because of their pandemic potential. Similar to seasonal flu, people with certain underlying conditions are at higher risk of developing serious complications from infections with swine-origin influenza viruses.

First case of A(H1N2)v virus in the U.S. in 2024

Every year, there are rare sporadic human infections with flu viruses that usually circulate in pigs. The first such infection with each novel influenza virus each year triggers international notification to global health authorities through International Health Regulations.

When a virus that normally spreads in pigs is found in people, these are called "variant flu virus" infections and are designated with the letter "v" after the subtype. Variant influenza A virus infections are novel influenza A virus infections. A novel influenza A virus is an influenza A virus that is different from seasonal influenza A viruses spreading among people.

Most human infections with variant influenza viruses occur following exposure to swine, but human-to-human spread can occur. However, in most cases, variant influenza viruses have not shown the ability to spread easily and sustainably from person to person. Variant influenza virus infections are usually associated with contact with pigs, often at agricultural fairs.

These infections are fully investigated to ensure that such viruses are not spreading efficiently in people and to limit further exposure of people to infected animals if infected animals are identified.

How variant infections occur

Human infections with variant influenza viruses happen mainly when an infected pig coughs or sneezes, and droplets containing influenza virus spread through the air. If these droplets land in your nose or mouth or are inhaled, you can be infected.

There is also some evidence that you might get infected by touching something that has a virus on it and then touching your own eyes, mouth, or nose. CDC has information about how variant virus infections happen.

The number of variant virus infections in the United States has ranged from a high of 321 during 2011–2012 to a low of one during 2018–2019 and 2019–2020.

CDC recommendations and guidance

Besides recommendations for people in contact with pigs or people who are at higher risk of serious flu complications, as well as recommendations for fair exhibitors, CDC has web resources and background to understand swine flu infections better. CDC also has information on prevention and treatment for people with exposure to pigs.