#16,265
Today's CDC's weekly FluView Report contains details on two novel flu infections - one from the 2020-2021 flu season (which ended with week 39), and one from the current (2021-2022) flu season. Both are swine variant viruses.While most swine variant infections can be traced to contract with pigs - either on farms or at agricultural exhibits - one of today's announced case does not appear to have had contact with swine, raising the possibility that it may have been transmitted from human-to-human.
While rare, we've seen other cases of limited H-2-H transmission with swine variants, and of course, the 2009 H1N1 swine flu was highly transmissible between humans. The CDC's risk assessment for Swine Variant viruses reads:
Sporadic infections and even localized outbreaks among people with variant influenza viruses may occur. All influenza viruses have the capacity to change and it’s possible that variant viruses may change such that they infect people easily and spread easily from person-to-person. The Centers for Disease Control and Prevention (CDC) continues to monitor closely for variant influenza virus infections and will report cases of H3N2v and other variant influenza viruses weekly in FluView and on the case count tables on this website
The risk of one of these swine variant viruses sparking a pandemic is relatively low, but it isn't zero. 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
Today's cases represent the 11th detected in the United States during the 2020-2021 flu season and the first case of the 2021-2022 flu season, although it is very likely that other cases have gone undetected. Most infections are mild, and most people will never be tested.
Novel Influenza A Virus
Two human infections with novel influenza A viruses were reported. One infection with an influenza A(H1N1) variant (A(H1N1)v) virus was reported by North Dakota that occurred during the 2020-21 influenza season and one infection with an influenza (A(H3N2)v) virus was reported by Ohio that occurred during the 2021-22 influenza season.
Both patients are <18 years of age, were not hospitalized, and have recovered or are recovering from their illness. One of the patients had close contact with swine prior to illness onset. The other patient had no known swine contact or attendance at agricultural exhibits where swine were present; it is possible that limited human-to-human transmission occurred. No ongoing human-to-human transmission has been identified associated with either patient.
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 close proximity 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.
During the 2021-22 influenza season, one human infection with novel influenza A virus has been reported in the United States: H3N2v (OH). During the 2020-21 influenza season, 11 human infections with a novel influenza A virus were reported in the United States, including two H3N2v (IA, WI), three H1N2v (IA, OH (2)), and six H1N1v (IA, NC, ND, WI (3)) virus infections.
Early identification and investigation of human infections with novel influenza A viruses are critical so that the risk of infection can be more fully understood and appropriate public health measures can be taken. Additional information on influenza in swine, variant influenza virus infection in humans, and strategies to interact safely with swine can be found at www.cdc.gov/flu/swineflu/index.htm.
Additional information regarding human infections with novel influenza A viruses can be found at http://gis.cdc.gov/grasp/fluview/Novel_Influenza.html.
For now, sporadic jumps of swine variant influenza from pigs to humans pose only a minor public health risk. But with every reassortment, and every species jump, these viruses are afforded new opportunities to overcome the barriers that have prevented them from becoming a pandemic strain.
And so we monitor there progress with considerable interest.
