At a glance
This CDC spotlight was originally posted on October 22, 2021. CDC reported two new U.S. human infections with a variant influenza virus that usually spreads in pigs and not people. In general, the risk to the public from these infections is considered low.
CDC Update
October 22, 2021—CDC today reported two new U.S. human infections with influenza viruses that usually spread in pigs and not people. These types of infections are called "variant influenza virus" infections and denoted with the letter "v" after the influenza subtype. One of these variant influenza virus infections is the first to occur during the 2021-2022 flu reporting season (i.e., from October 2021 through September 2022) and the other reported infection is the 11th infection that occurred during the 2020-2021 season (i.e., from October 2020 through September 2021). These infections serve as a reminder of the importance of following CDC's recommended precautions around swine as well as the importance of ongoing surveillance to detect potential pandemic viruses.
Variant virus infections occur rarely, and usually in the context of exposure to pigs when they happen. However, there have been a small proportion of such infections where the original source of exposure to a pig cannot be readily identified. When this happens, the possibility of limited human-to-human spread of a variant influenza virus cannot be ruled out. That means the virus may have passed from a pig to an intermediary person who then spread the virus to the patient in whom the virus was detected. There is epidemiologic follow-up for every case reported to try to identify any such intermediary persons and to make sure there is no ongoing spread of the virus in question. Of the two just-reported variant influenza virus infections, one of the patients had contact with pigs prior to illness onset; but in the other case, no known direct or indirect swine exposure has been identified. It is possible that limited human-to-human spread occurred in this case, however, no ongoing human-to-human spread has been identified (with either patient).
There are three main groups of influenza viruses that commonly spread among pigs in the United States: H1N1, H1N2, and H3N2 viruses. These viruses have important antigenic and genetic differences from seasonal influenza A viruses that circulate worldwide among people. The two newest infections, one an H3N2v virus and the other an H1N1v virus, occurred in children. Neither child was hospitalized, and both have recovered or are recovering from their illness.
Illnesses associated with variant influenza virus infections have been mostly mild with symptoms similar to those of human seasonal influenza. However, variant influenza virus infections also can cause serious illness, resulting in hospitalization and death. Seasonal flu vaccines are not formulated to protect against variant influenza viruses, but the same flu antiviral drugs used to treat seasonal flu can be used to treat variant influenza virus infection in children and adults.
In 2005, these infections became nationally notifiable. Since that time, a total of 497 variant influenza virus infections (of different influenza A virus subtypes) have been identified in the United States and reported to CDC ranging from a high of 321 variant influenza virus infections during the 2011-2012 flu season to a low of one during the 2018-2019 and 2019-2020 seasons The 321 infections reported during the 2011-2012 seasons included 315 H3N2v, four H1N2v, and two H1N1v viruses detected during the 2011-2012 influenza season. More than 90% of those infections were associated with attendance at agricultural fairs. Following that season, CDC implemented significant education and outreach efforts to raise awareness about the public health concerns related to exposure to pigs. Since then, identification of variant influenza virus infections has been less common, particularly for those associated with agricultural events.
Influenza viruses can spread from pigs to people and from people to pigs. Infected pigs can cough or sneeze and droplets with influenza virus in them can spread through the air. If these droplets land in your nose or mouth, or are inhaled, you can be infected. Pigs can be infected by avian influenza and human influenza viruses as well as swine influenza viruses. When influenza viruses from different species infect pigs, the viruses can reassort (i.e., swap genes) and new viruses that are a mix of swine, human and/or avian influenza viruses can emerge. This is thought to have happened in 2009 when a new H1N1 virus with genes of avian, swine and human origin emerged to cause a flu pandemic.
CDC has guidance for people who work or interact with pigs and for people attending fairs where pigs might be present, including additional precautions for people who are at increased risk of serious flu complications. In general, the risk to the public from these infections is considered low, but each case of human infection with a variant influenza virus should be fully investigated to be sure that such viruses are not spreading in an efficient and ongoing way in humans, and limit further exposure of humans to infected animals if infected animals are identified. CDC reports these cases in FluView.