#18,233
While most of our attention has been focused on avian flu this summer, since 2010 we've seen more than 500 scattered reports of infection with swine variant influenza viruses (H1N1v, H1N2v & H3N2v) around the nation, often associated with agricultural exhibits at county and state fairs.
Swine variant influenza infections are indistinguishable from seasonal flu without highly specialized testing, which few countries tend to do. Even here in the United States it is assumed we only detect a small percentage of cases (see CID Journal: Estimates Of Human Infection From H3N2v (Jul 2011-Apr 2012)).
Likely because they are doing extra testing due to their recent spate of H5N1 cases (n=10), Colorado has picked up a swine variant infection, which is reported in the latest CDC FluView report (see below).
Novel Influenza A Virus:
A human infection with a novel influenza A virus was reported by the Colorado Department of Public Health and Environment. The patient was infected with an influenza A(H3N2) variant (A(H3N2)v) virus. The patient is < 18 years of age, sought healthcare during the week ending July 13, 2024 (Week 28), and was not hospitalized. An investigation by state public health officials found the patient attended an agricultural event prior to their illness onset. Investigation did not identify illness among close contacts of the patient. The investigation is ongoing.
Three cases of influenza A (H1N2)v were also reported during the 2023-2024 season bringing the total to 4 variant influenza cases this season. No new human infections of avian influenza A(H5N1) virus have been reported during Week 31. A total of 13 cases of human infection with influenza A (H5) were previously reported this season bringing the total to 17 novel influenza cases reported during the 2023-2024 season. An ongoing outbreak of H5N1 continues in domestic dairy cattle and poultry and monitoring for additional cases is ongoing.
When an influenza virus that normally circulates in swine (but not people) is detected in a person, it is called a “variant” influenza virus. Most human infections with variant influenza viruses occur following exposure to swine, but human-to-human transmission can occur. It is important to note that in most cases, variant influenza viruses have not shown the ability to spread easily and sustainably from person to person.
Early identification and investigation of human infections with novel influenza A viruses are critical so that the risk of infection can be understood, and appropriate public health measures can be taken.
Additional information on influenza in swine, variant influenza virus infection in humans, and guidance to interact safely with swine can be found at www.cdc.gov/flu/swineflu/index.htm.
With few exceptions (i.e. the 2009 H1N1 pandemic virus), current swine influenza viruses don't appear to spread efficiently in humans. That said, the CDC's IRAT (Influenza Risk Assessment Tool) lists 3 North American swine viruses as having at least some pandemic potential (2 added in 2019).
H1N2 variant [A/California/62/2018] Jul 2019 5.8 5.7 Moderate
H3N2 variant [A/Ohio/13/2017] Jul 2019 6.6 5.8 Moderate
H3N2 variant [A/Indiana/08/2011] Dec 2012 6.0 4.5 Moderate
The CDC currently ranks a Chinese Swine-variant EA H1N1 `G4' as having the highest pandemic potential of any flu virus on their list. But, as with avian flu, they all have barriers they must first overcome.
Although the public health risks are considered low, the CDC advises those who are at higher risk of serious flu complications (including children under 5, adults over 65, pregnant women, and those with certain chronic medical conditions), to avoid pigs and the swine barn altogether.
