Situation Summary on Influenza A (H3N2) Variant Viruses (“H3N2v”)

On This Page

Background

Swine flu viruses do not normally infect humans. However, sporadic human infections with influenza viruses that normally circulate in swine and not people have occurred. When this happens, these viruses are called “variant viruses.” They also can be denoted by adding the letter “v” to the end of the virus subtype designation. Human infections with H1N1v, H3N2v and H1N2v viruses have been detected in the United States. Most commonly, human infections with variant viruses occur in people with exposure to infected pigs (e.g., children near pigs at a fair or workers in the swine industry). This is thought to happen mainly when an infected pig coughs or sneezes and droplets with influenza virus in them spread through the air. If these droplets land in your nose or mouth, or are inhaled, you can be infected. There also is some evidence that you might get infected by touching something that has virus on it and then touching your own mouth or nose. A third way to possibly get infected is to inhale particles containing influenza virus. Scientists aren’t really sure which of these ways of spread is the most common. Swine influenza has not been shown to be transmissible to people through eating properly handled and prepared pork (pig meat) or other products derived from pigs.

CDC Assessment

It’s possible that sporadic infections and even localized outbreaks among people with this virus may occur. While there is no evidence at this time that sustained human-to-human transmission has occurred, all influenza viruses have the capacity to change and it’s possible that this virus may change and become widespread in people. Illness associated with H3N2v infection so far has been mostly mild with symptoms similar to those of seasonal flu. Like seasonal flu, however, serious illness, resulting in hospitalization and death is possible. In 2012, for example, of 309 human infections with H3N2v, 16 people were hospitalized and one of these people died. Most of the people who were hospitalized and the person who died had one or more health or age factor that put them at high risk of serious flu-related complications. People at high risk of serious complications from seasonal influenza and H3N2v include children younger than 5, people with certain chronic conditions like asthma, diabetes, heart disease, weakened immune systems, pregnant women and people 65 years and older. CDC has issued guidance for people attending fairs where swine might be present, including additional precautions for people who are at high risk of serious flu complications. Limited serologic studies indicate that adults may have some pre-existing immunity to this virus while children do not. Most cases of H3N2v infection have occurred in children who have little immunity against this virus.

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.