CDC Reports Two Human Infections with Variant Influenza Viruses

CDC Update

June 28, 2024 — CDC has reported this year's second and third U.S. human infections with influenza (flu) viruses that normally spread in pigs and not in people. These infections with influenza A(H1N2) variant (v) viruses occurred in two people in Pennsylvania who attended a livestock auction where pigs were present. 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 patients, both adults, are close contacts. Both sought medical care for their illnesses during the week ending June 22. One of the patients was hospitalized but has since been discharged, and both patients are recovering. An investigation by state health officials in Pennsylvania did not identify additional illness among close contacts of either patient. This investigation is ongoing.

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 variant influenza viruses.

Second and third cases of A(H1N2)v virus in the United States in 2024

Every year, there are rare, sporadic human infections with influenza viruses that usually circulate in pigs and not in people.

When a virus that normally spreads in pigs is found in people, these are called "variant influenza 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 the seasonal influenza A viruses spreading among people.

Most human infections with variant influenza viruses occur following exposure to infected swine, but limited person-to-person spread has occurred. However, in most cases, variant influenza viruses have not shown the ability to spread easily and sustainably from person to person. Many variant influenza virus infections have occurred in people who have had contact with pigs at agricultural fairs.

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

These variant virus infections are unrelated to the ongoing outbreak of H5N1 bird flu in poultry and dairy cows.

How variant influenza virus infections happen

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 more information about how variant virus infections happen.

The number of variant influenza virus infections identified 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. The first H1N2v virus infection of 2024 was also detected in Pennsylvania and was reported at the end of March. That person lived near a pig farm and had direct contact with pigs.

CDC Recommendations and Guidance

CDC has provided guidance for clinicians on the identification and treatment of variant influenza infections in people, recommendations for people in contact with pigs or people who are at higher risk of serious flu complications, and recommendations for fair exhibitors and organizers. CDC also has web resources and background information to help better understand swine flu infections.