#17,533
The CDC maintains a growing list 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.
- Potential Emergence Risk: The likelihood that a particular virus will become transmissible among humans
- Potential Impact Risk: And its impact (based on virulence, population immunity, etc.), should it become a pandemic strain
While IRAT can't predict which virus will become a pandemic strain (or when), it can help planners decide which viruses pose the greatest risks, so they can prioritize their efforts and investments.
This list has expanded rapidly since 2011, with 20 of 24 current flu subtypes added since then.
Until today, the most recent - H5N1: clade 2.3.4.4b: [A/American wigeon/South Carolina/AH0195145/2021] - was added in May of 2022, a few months after H5N1 arrived in the United States.
As we've seen (see H5N1: The Only Constant is Change), this H5 virus has evolved into dozens of different genotypes and variants, both here in the U.S., and around the globe. But the most concerning to date has been the rapid spread of a mutated HPAI A(H5N1) Virus in Farmed Minks, Spain, October 2022.
Today the CDC has announced a new assessment of the mink-variant H5N1 virus in Spain. While still classified as a `moderate' threat, it's scores have risen in 6 of the 10 parameters used to evaluate their zoonotic potential (see chart below).
The CDC offered the following prelude to the new assessment:
On July 4, 2023, CDC published a new IRAT assessment for A(H5N1) clade 2.3.4.4b [A/mink/Spain/3691-8_22VIR10586-10/2022]. This virus was associated with an outbreak among mink at a mink farm in Spain in 2022. The H5N1 bird flu viruses that are causing outbreaks in wild birds and poultry, occasional infections in mammals, and rare human infections are H5N1 clade 2.3.4.4b viruses. A risk assessment for the potential ‘emergence’ and ‘public health impact’ for those viruses was originally conducted in March 2022 using A/American wigeon/South Carolina/AH0195145/2021.
This updated assessment includes new information available since then, including from eight additional human cases. This updated assessment using the mink virus indicates that this virus has scored slightly higher in some risk elements compared with the previously assessed H5N1 clade 2.3.4.4b virus isolated from an American wigeon duck in 2022. However, the mean-high and mean-low acceptable score ranges for these viruses overlap, indicating that these viruses remain similar, and their overall risk scores remain “moderate.” New data visualizations have been added to this page to aid in the comparison of these viruses.
The summary of their new assessment follows:
Since January 2022, highly pathogenic avian influenza A(H5N1) virus (AIV) clade 2.3.4.4b 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 multiple states.
The A(H5N1) clade 2.3.4.4b 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 2.3.4.4b viruses by 2021. This virus was first reported in migrating wild aquatic birds in North America in December 2021 and in South America in January 2023. In 2023, the virus has been detected in more than 50 countries in these five regions. The first detections in the United States were from migratory wild aquatic birds but the virus has also been detected in domestic poultry. Sporadic incursions of the virus into aquatic and terrestrial carnivorous mammals have occurred in the United States and other regions, including an outbreak in a farmed mink unit in Spain.
Between 2022-2023, nine cases of human infection with A(H5N1) clade 2.3.4.4b viruses have been identified in Europe, Asia, and the Americas. Five of these cases resulted in severe/critical lower respiratory tract disease with one fatality.
Phylogenetic analysis of A(H5N1) clade 2.3.4.4b viruses shows high levels of genetic similarity to previously circulating A(H5Nx) clade 2.3.4.4b viruses, with little evidence of mammalian adaptation. The hemagglutinin (HA) genes of currently circulating wild bird and poultry A(H5N1) clade 2.3.4.4b viruses show a high level of genetic similarity to previous clade 2.3.4.4 viruses, with some genetic variation noted among the N1 neuraminidase (NA) gene, which is wild bird adapted.
Previously recommended A(H5) candidate vaccine viruses (CVVs) are expected to be effective against A(H5N1) viruses currently circulating among wild birds and poultry. In addition, A(H5N1) virus genetic analysis suggests that a majority of viruses remain susceptible to available influenza antiviral medications.
Summary: A re-evaluation of the risk assessment of highly pathogenic avian influenza A(H5N1) virus clade 2.3.4.4b, using A/mink/Spain/3691-8_22VIR10586-10/2022 as the new prototype strain, was conducted in April 2023. 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 5.13, in the mid-low range of the moderate risk category. The average risk score for the virus to potentially impact public health was 6.24, in the mid-range of the moderate risk category. These scores reflect an increase of 0.69 in the emergence question and an increase of 1.17 in the impact question compared to the previous A(H5N1) evaluation last year, but both questions still fall into the moderate risk category.
The mink virus scores higher than the previously assessed American wigeon virus across 6 out of the 10 risk elements evaluated by the IRAT, including the following: antiviral treatment options, disease severity and pathogenesis, genomic analysis, human infections, infections in animals, and transmission in animal models. These increased scores reflect new information about HPAI H5 2.3.4.4b viruses, including seven new human detections–many with severe clinical courses. Additionally, there have been sporadic infections and sustained transmission in wild birds and domestic poultry in Asia, Europe, Africa, and the Americas, including multiple detections in mammals with genetic markers of importance for mammalian/human infection (but not in birds). A bar graph comparing each of the 10 weighted average risk elements scores for the mink and American wigeon virus is available Summary of Influenza Risk Assessment Tool (IRAT) Results. The average subject matter expert (SME) confidence level in the available data of all 10 risk elements was 2.32 (SME confidence range: 1.0-4.0). The full report [486 KB, 8 Pages] is available.
Flu Date: Apr 1, 2023
While the zoonotic risk may have increased incrementally with this particular variant, the good news is this virus obviously still needs to make some genetic improvements before it can become a genuine pandemic threat.
The bad news is, that it - along with dozens of other genotypes circulating around the globe - keep trying.
