The CDC monitors a (growing) number of novel influenza viruses with zoonotic potential on their IRAT (Influenza Risk Assessment Tool) website - that, using a variety of parameters - attempts to quantify their threat in two broad categories.
In the past few days, with little fanfare, the CDC added the 23rd virus to this list; Avian A/H5N1 Clade 188.8.131.52b, following reports of the first human infection emerged from the UK earlier this year. This report was finalized in March, before the 1st Human HPAI H5 Infection was reported in the United States.
The CDC ranks the Avian A/H1N1 virus's threat in the lower-middle of the pack, well below China's EA H1N1 swine-variant virus, or the far more virulent HPAI H5N6, or H7N9 viruses. The CDC's summary follows:
Since January 2022, highly pathogenic avian influenza A(H5N1) virus clade 184.108.40.206b has been detected in the United States in numerous wild bird species, including aquatic birds such as ducks, and in commercial and backyard domestic poultry in numerous states.
The A(H5N1) clade 220.127.116.11b virus emerged in 2020, spreading across Europe, Asia, and Africa, in both wild aquatic birds and domestic poultry, and replacing the previously circulating A(H5N8) clade 18.104.22.168b viruses by 2021. In 2022, the virus has been detected in more than 50 countries in these three regions and was first reported in migrating wild aquatic birds in the Americas in December 2021. First detections in the United States also were from migratory wild aquatic birds.
In January 2022, one asymptomatic human infection with this virus, with known repeated exposure to infected ducks in England, was reported to the World Health Organization.
Phylogenetic analysis of A(H5N1) clade 22.214.171.124b viruses show high levels of genetic similarity to previously circulating A(H5Nx) clade 126.96.36.199b viruses, with little to no evidence of mammalian adaptation. The hemagglutinin genes of circulating wild bird and poultry viruses show a high level of genetic similarity, with some genetic variability noted among the neuraminidase gene, which is wild bird adapted.
Previously recommended A(H5) candidate vaccine viruses are expected to be effective against A(H5N1) viruses currently circulating among wild birds and poultry. In addition, A(H5N1) virus genetic and phenotypic analysis suggests that they remain susceptible to available influenza antiviral medications.
Summary: A risk assessment of highly pathogenic avian influenza A(H5N1) virus clade 188.8.131.52b, using A/American wigeon/South Carolina/AH0195145/2021 as the prototype strain, was conducted in March 2022. The overall estimated IRAT scores placed this virus in the moderate risk category, which ranges from 4.0 to 7.9. The average risk score for the estimated potential emergence of the virus was 4.4, in the lower range of the moderate risk category. The average risk score for the virus to potentially impact public health was 5.1, also in the lower range of the moderate risk category. The average confidence level in the available data of all 10 risk elements was 2.4 (range: 1.0-3.0).
And as we've discussed often over the past few years (see Are Influenza Pandemic Viruses Members Of An Exclusive Club?), the progression of human influenza pandemics over the past 130 years has been H2, H3, H1, H2, H3, H1, H1 . . . .
Simply put, novel H1, H2, and H3 flu viruses appear to have fewer barriers to overcome in order to jump to humans. That doesn't mean that an H5 or H7 virus can't adapt well enough to start a pandemic, only that we've never seen it happen (yet).