CDC Reports Eighth and Ninth U.S. Human Infections with a Variant Flu Virus for 2020-2021 Influenza Season

CDC Update

September 10, 2021—CDC today reported the eighth and ninth U.S. human infections with influenza viruses that usually spread in pigs and not people occurring during the 2020-2021 influenza season (i.e., from October 2020 through September 2021). When people are infected with a flu virus that normally spreads in pigs these are called "variant influenza virus" infections and denoted with the letter "v". These types of infections occur in people rarely, and usually in the context of contact with pigs but are concerning because of their pandemic potential. These infections also serve as a reminder of the importance of following CDC's recommended precautions for people to take when around swine.

There are three main groups of flu 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 H1N2v virus, occurred in children. Both children had either direct contact with pigs or had a household member who had direct contact with pigs prior to illness onset. No person-to-person spread of variant influenza virus was identified associated with either of these patients, and neither of the children were hospitalized. The previous seven of these infections occurred in people who also reported that they had direct contact with pigs or lived on a property where pigs were present.

The state breakdown for the nine variant infections reported during 2020-2021 is: Iowa (one H1N1v, one H1N2v, and one H3N2v), North Carolina (one H1N1v), Ohio (one H1N2v), and Wisconsin (one H3N2v and three H1N1v). Five of these infections were in children younger than 18 years, and four occurred in adults. Each patient fully recovered from their illness, and no person-to-person spread of variant influenza virus was identified associated with any of these patients.

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. These infections have most commonly been reported after close proximity to infected pigs, such as in pig barns and livestock exhibits housing pigs at fairs. Like influenza viruses in humans and other animals, swine flu viruses change constantly. 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.

Agricultural fairs take place across the United States every year, primarily during the summer months and into early fall. Many fairs have swine exhibitions, where pigs from different places come into close contact with each other and with people. These venues may increase the risk of spread of influenza viruses between pigs and people. In order to reduce the risk of variant influenza virus infection to people posed by interactions between people and pigs at fairs, CDC recommends that people at higher risk for serious flu complications avoid pigs and swine barns at fairs. CDC has issued guidance for people attending fairs where pigs might be present, including additional precautions for people who are at increased risk of serious flu complications.

Prevention measures to limit the spread of influenza viruses include but are not limited to the following: not eating or drinking while in pig areas, avoiding contact with pigs that appear to be sick, and washing your hands often with soap and running water before and after contact with pigs.

Since 2005, a total of 493 variant influenza virus infections (of all influenza A virus subtypes) have been identified in the United States and reported to CDC. There has been some limited, non-sustained, person-to-person spread of variant influenza viruses, but no ongoing community transmission has been identified outside of the 2009 H1N1 pandemic. Illnesses associated with variant influenza virus infections have been mostly mild with symptoms similar to those of human seasonal flu. 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 viruses, but the same influenza antiviral drugs used to treat seasonal flu can be used to treat variant influenza virus infection in children and adults.

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 its weekly national influenza surveillance report, FluView.