CDC Confirms Two New Human Infections with Flu Virus from Pigs During 2022
August 12, 2022—CDC has reported two new human infections with an influenza (flu) virus that usually spreads in pigs, bringing the total number of such infections in the United States during 2022 to three. These two new infections were in people who attended the same West Virginia agricultural fair as the first variant flu infection of 2022 reported by CDC on August 5. Sporadic human infections with these flu viruses that usually spread in pigs happen every year, often in the agricultural fair setting, which are typically held in the summer and fall. CDC recommends people take precautions around pigs, including in the fair setting.
When flu viruses that usually spread in pigs are found in people, they are called “variant flu virus” infections and designated with the letter “v” after the subtype. While these types of infections usually cause mild illness, they are concerning because they can cause severe illness, especially in people at higher risk of serious flu complications, and because of their potential to cause a flu pandemic.
An investigation into this situation has found that:
- Pigs at the fair tested positive for swine influenza A(H3N2) virus;
- All three human infections were with flu A(H3N2)v viruses based on RT-PCR testing and virus sequence analysis done at CDC.
- CDC genetic sequencing data indicates that the three flu viruses found in specimens obtained from the infected people are more than 99% identical to those obtained from infected pigs at the same fair.
- CDC will conduct additional laboratory tests on these three variant flu viruses, including analysis to determine whether flu vaccines stockpiled in case of a future flu pandemic or seasonal flu vaccines would protect against infection and severe illness with these viruses.
- All three infections were in people younger than 18 years.
- None of the patients were hospitalized and all have recovered from their illnesses.
- Two of the three infected people had direct contact with pigs.
In a small proportion of these types of infections, the original source of exposure to a pig cannot be readily identified. In these cases, the possibility of limited person-to-person spread cannot be ruled out. Routine follow-up has not found any ongoing person-to-person spread of this virus.
Take Precautions while Attending Agricultural Fairs
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 flu viruses among pigs and between pigs and people due to these interactions.
- People who are at higher risk for developing serious flu complications should avoid pigs and swine barns at fairs.
- If they cannot avoid exposure to pigs, they should wear a well-fitting mask that covers the nose and mouth to reduce their risk of exposure to flu viruses.
- They should also wash their hands with soap and running water before and after exposure to pigs or a swine barn. If soap and water are not available, they should use an alcohol-based hand rub.
For people who are not at higher risk of serious flu complications, prevention measures to limit the spread of flu viruses include:
- Not eating or drinking while in pig areas,
- Avoiding contact with pigs that appear to be sick, and
- Washing hands often with soap and running water before and after contact with pigs.
People should take additional protective measures if they must come in contact with pigs that are known or suspected to be sick. This includes minimizing contact time with pigs and wearing personal protective equipment like protective clothing, gloves, and well-fitted masks that cover the mouth and nose when contact is required.
Note that seasonal flu vaccines are not formulated to protect against variant flu viruses, but the same flu antiviral drugs used to treat seasonal flu can be used to treat variant flu virus infection in children and adults.
How Variant Flu Virus Infections Happen
Flu viruses can spread from pigs to people and from people to pigs. Infected pigs can cough or sneeze, and droplets with flu 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.
Flu Viruses From Pigs Change Constantly
Like flu viruses in humans and other animals, flu viruses found in pigs change constantly. Pigs can be infected by avian flu and human flu viruses as well as their own flu viruses. When flu viruses from different species infect pigs, the viruses can reassort (i.e., swap genes) and new viruses can emerge that can infect and spread easily from person-to-person. 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.
In 2005, human infection with a novel flu A virus flu became nationally notifiable in the United States. Novel flu A viruses are different from current seasonal viruses circulating in people and include variant flu viruses and avian flu viruses. Since that time, a total of 503 variant flu virus infections (of different flu A virus subtypes) have been identified in the United States and reported to CDC ranging from a high of 321 variant flu 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 A(H3N2)v, four A(H1N2)v, and two A(H1N1)v viruses detected during the 2011-2012 flu season. More than 90% of those infections were associated with attendance at agricultural fairs.
In general, the risk to the public from these infections is considered low, but each case of human infection with a variant flu virus should be fully investigated to be sure that such viruses are not spreading in an efficient and ongoing way in people, and to limit further exposure of people to infected animals, if infected animals are identified. CDC is monitoring this situation closely and will make adjustments to the public health risk assessment and recommendations as circumstances warrant. CDC reports these cases in FluView.