For the past decade, during the summer months, we are on watch for reports of novel swine flu infections in humans, which are usually associated with county or state fair attendance or the raising of pigs.
Since 2005, over 465 human infections (H1N1v, H1N2v or H3N2v) have been documented in the United States, with over 300 of those reported in 2012.
Surveillance for these types of viruses is extremely limited, and it is likely far more cases occur than are reported. Earlier this year, in WHO: Influenza A(H1N2) variant virus – Brazil we looked at a recent case reported from South America, and in mid July the WHO reported on a case in Germany.
This summer, with agricultural fairs shuttered, we've not seen any U.S. cases. That is, until today, as the CDC reports on a H3N2v infection in a child living in Hawaii. Of note this case does not appear to have had direct contact with swine, and an epidemiological investigation is ongoing.
Novel Influenza A Virus
One human infection with a novel influenza A virus was reported by Hawaii. This person was infected with an influenza A(H3N2) variant (A(H3N2)v) virus. The patient is a child < 18 years of age, was not hospitalized, and has recovered from their illness. While no exposure to swine has been reported to date, an investigation is ongoing into the source of the patient’s infection. This is the first influenza A(H3N2)v virus infection detected in the United States since 2018.
Influenza viruses that circulate in swine are called swine influenza viruses when isolated from swine, but are called variant viruses when isolated from humans. Seasonal influenza viruses that circulate worldwide in the human population have important antigenic and genetic differences from influenza viruses that circulate in swine.
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 infection in humans, and strategies to interact safely with swine can be found at http://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.
While most swine variant infections don't appear to transmit well in humans, the CDC's IRAT (Influenza Risk Assessment Tool) lists 3 North American swine viruses as having some pandemic potential (2 added in 2019).
H1N2 variant [A/California/62/2018] Jul 2019 5.8 5.7 ModerateH3N2 variant [A/Ohio/13/2017] Jul 2019 6.6 5.8 ModerateH3N2 variant [A/Indiana/08/2011] Dec 2012 6.0 4.5 Moderate
And we follow a number of other swine variants around the world with similar potential. Recently China's EA H1N1 `G4' virus has garnered a lot of attention (see ECDC Risk Assessment: Eurasian avian-like A(H1N1) swine influenza viruses), as have other swine around the globe.
And as we've discussed many times before (see Are Influenza Pandemic Viruses Members Of An Exclusive Club?) - most swine influenza viruses are either H1, H2, or H3 - giving them a distinct advantage, as those are the only influenza subtypes known to have sparked a human pandemic in the last 130 years.
So far, the good news is that most swine variant viruses haven't become biologically `fit' enough spark a pandemic. In order to be successful, they need to be able to replicate and transmit on par with already circulating human flu viruses.
But of course, we saw exactly that happen in 2009 with a swine variant H1N1, and so we watch these sporadic species jumps with considerable interest.