#18,465
Last August the CDC released an IRAT Risk Assessment On A/Texas/37/2024 H5N1 Virus - the 3rd H5N1 addition (all clade 2.3.4.4b) to the IRAT (Influenza Risk Assessment Tool) in just over 3 years.
That analysis compared the `bovine' B3.13 strain to the Spanish `Mink Strain' added in 2023, and a strain collected in 2022 in South Carolina from a Wigeon. The CDC summarized the findings as:
This IRAT assessment indicates that while the virus that caused an H5N1 bird flu infection in a person in Texas [A/Texas/37/2024 virus] in 2024 scored slightly lower in some risk elements and slightly higher in others compared with two previously assessed avian influenza A(H5N1) clade 2.3.4.4b viruses, the mean-high and mean-low acceptable score ranges for these viruses overlap. This means that the future pandemic potential risk from these viruses remains similar and is classified as "moderate" risk.
Their assessment puts A/Texas/37/2024 8th from the top, in terms of risk, with China's Swine-variant EA H1N1 `G4' virus - which we've been following since 2015 - still firmly at the top of the list.
This week's new IRAT adds two more strains:- A/California/147/2024 genotype B3.13
- A/Washington/239/2024 genotype D1.1
Both of which rank slightly below the Texas strain reported last year.
The full Influenza Risk Assessment Tool (IRAT) - Virus Report (March 14th, 2025) runs 13 pages, but you'll find excerpts from the CDC summary below:
Results of Influenza Risk Assessment Tool
At a glance
- The Influenza Risk Assessment Tool (IRAT) is a CDC evaluation tool developed with assistance from global animal and human health influenza experts.
- The IRAT is used to assess the potential pandemic risk of influenza A viruses that are not currently circulating in people.
- This latest IRAT assessed two recent clade 2.3.4.4b avian influenza A(H5N1) viruses: A/California/147/2024 and A/Washington/239/2024.
- These viruses scored in the "moderate risk" category for potential emergence and public health impact, similar to previous assessments of earlier clade 2.3.4.4b avian influenza A(H5N1) viruses. These results validate the proactive, coordinated U.S. government response.
- The IRAT does not assess the immediate risk to the public's health, which is unchanged and remains low, and it does not predict future pandemics.
(SNIP)
Key findings
On May 2, 2025, CDC published a new IRAT assessment for two clade 2.3.4.4b avian influenza A(H5N1) viruses: A/California/147/2024 and A/Washington/239/2024. A/California/147/2024 is a B3.13 genotype virus, like the ones currently circulating in dairy cows in the United States and causing sporadic human infections, mostly among people who had exposure to H5N1 virus-infected or presumed infected dairy cattle. A/Washington/239/2024 is a D1.1 genotype virus, like those that are most commonly circulating in wild/migratory birds and also causing poultry outbreaks and sporadic human illnesses, mostly among people who had exposure to poultry confirmed to have influenza A(H5N1) virus infection. The genes of this virus are more closely related to what has been circulating most commonly in U.S. wild birds and poultry.
Previously, CDC assessed three other clade 2.3.4.4b avian influenza A(H5N1) viruses, including A/American wigeon/South Carolina/AH0195145/2021, A/mink/Spain/3691-8_22VIR10586-10/2022, and A/Texas/37/2024. All three previously assessed viruses had overall estimated IRAT scores in the moderate risk category range of 4.0 to 7.9.
This updated assessment includes new information, including information from additional human cases reported in the United States. This updated assessment indicates that these two viruses (A/California/147/2024 and A/Washington/239/2024) scored slightly lower in some risk elements and slightly higher in others compared with the previously assessed H5N1 clade 2.3.4.4b viruses.
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."
- The average risk scores for the potential emergence of the A/California/147/2024 and A/Washington/239/2024 viruses were 5.59 and 5.21, respectively, placing them in the mid-low range of the moderate risk category.
- The average risk scores for these two viruses to potentially impact public health was 5.91 and 6.00, respectively, placing them in the mid-range of the moderate risk category.
These scores reflect a decrease of at least 0.20 in the potential emergence question and a decrease of at least 0.09 in the potential public health impact question compared with the previous A/Texas/37/2024 virus evaluation from last year, but both questions on emergence and public health impact still fall into the moderate risk category.
Some variation was seen among subject matter expert (SME) point estimate scores across the risk elements, including Human Infections and Infections in Animals, where the scores ranged from moderate to high risk for the A/California/147/2024 virus, and Disease Severity and Pathogenesis, Global Distribution of Animal Influenza Viruses, and Human Infections for the A/Washington/239/2024 virus. This indicates some uncertainty in interpretation and confidence of the available data.
Sensitivity analyses using the lowest and highest scores for these four risk elements resulted in adjusted ranges for the overall emergence risk and the potential impact risk that continued to place this virus in the mid-range of the moderate risk category. This indicates that the categorization of HPAI A(H5N1) clade 2.3.4.4b virus, including A/California/147/2024 and A/Washington/239/2024, as moderate risk was unchanged by the range of scores within the risk elements exhibiting variation.
While IRAT can't tell us which virus will spark the next pandemic, or when that might happen, there are three major takeaways from these IRAT reports.
- There are a lot of novel flu threats out there with at least some pandemic potential. IRAT currently evaluates 27 (up from 4 in 2012), but that is by no means an exhaustive list.
- New viruses appear to be emerging at an accelerated rate over the past decade.
- Viral evolution requires that these viruses be re-evaluated from time to time, as nothing is static in the flu world. A virus that ranks relatively low today might be viewed as a serious contender a year or two from now.
All reasons why we need to be prepared for surprises.
