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.
Purpose
The IRAT uses expert opinion to evaluate the potential of a representative novel influenza A virus to gain the ability for person-to-person spread and the resulting potential public health impact if that were to happen, and it allows for comparison to other viruses evaluated in past IRAT reports.
The IRAT does not assess the immediate risk to the public's health, and it does not predict future pandemics.
The immediate risk to the public's health from clade 2.3.4.4b avian influenza A(H5N1) viruses is unchanged and is currently considered to be low.
The current H5N1 bird flu situation continues to be mostly an animal health issue.
Human infections with avian influenza A(H5N1) viruses are rare, and these viruses still are not well-adapted to spreading among people, as they do not currently have the ability to easily infect the human upper respiratory tract.
Of the 70 cases in the United States from 2024-2025, 41 were associated with exposure to influenza A(H5N1)-infected cattle and 24 were associated with exposure to influenza A(H5N1)-infected poultry/birds.
In addition, there were two cases with backyard poultry exposure and three cases with unknown exposure.
Globally, most past human infections with avian influenza A(H5N1) viruses have occurred following close, unprotected contact with sick or dead birds.
This report summarizes the findings of an IRAT conducted on two viruses that caused recent human illnesses:
-- A/California/147/2024
and
-- A/Washington/239/2024.
The resulting score places both of these viruses, representative of two main groups of avian influenza A(H5N1) viruses currently circulating among animals in the United States, in the category of "moderate risk" for potential future emergence and public health impact. This is similar to previous assessments of earlier avian influenza A(H5N1) viruses. The scores for this IRAT were submitted on March 14, 2025.
During a public health response, the IRAT can be used to assess the appropriateness of the ongoing response efforts and whether additional actions are warranted based on the risk score.
The results of this IRAT validate the proactive, coordinated U.S. government response. Assessing risk is an iterative process with new information being assimilated regularly and response activities adjusted as indicated.
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.
The full report is available at Virus Descriptions and Report Summaries.
Background
Input on IRAT assessments is provided by a diverse group of U.S. government animal and human health influenza experts. More information about the IRAT, including a description of its methodology and definitions for its risk elements and categories, is available at Influenza Risk Assessment Tool (IRAT). The IRAT is updated when new zoonotic or novel influenza A viruses with pandemic potential emerge or undergo a change in characteristics that prompts the need for a new assessment.
Summary of U.S. Human Cases of H5 Bird Flu
In the United States since April 2024, 70 human illnesses with H5 bird flu have been reported in 13 states. One additional human case of H5N1 bird flu was reported in the United States in April 2022 in a farm worker who experienced fatigue without any other symptoms and while depopulating poultry at a poultry farm with confirmed avian influenza A(H5N1) virus.
Summary of U.S. human cases associated with dairy cattle exposure
The first human case in the United States associated with the outbreaks of A(H5N1) virus among dairy cattle was reported on April 1, 2024, by the State of Texas. As of April 2024, 41 human cases of H5 bird flu have been associated with the ongoing multi-state outbreak of A(H5N1) in dairy cattle, with 36 cases occurring in California, two cases in Michigan, and one case each reported in Colorado, Nevada and Texas. All infections occurred in dairy workers who had direct exposure to cattle presumed to be infected with A(H5N1) virus. Infections associated with U.S. dairy cattle to date have involved mild respiratory symptoms or conjunctivitis. No patients have been hospitalized.
Summary of U.S. human cases associated with poultry or backyard flock exposure
Twenty-four human cases of H5 bird flu have been detected in farm workers who were involved in the depopulation of poultry at a poultry facility experiencing an outbreak of HPAI A(H5N1) virus, and two cases involved exposures to backyard flocks in Louisiana and Wyoming. Eleven of the 24 cases in farm workers were reported in Washington state, nine in Colorado, and one each in Iowa, Ohio, Oregon, and Wyoming. Most of these workers who tested positive reported mild illness, such as redness/watery eyes and respiratory symptoms. However, one case in Ohio was severe and required hospitalization; the patient subsequently recovered. In addition, the two cases exposed to backyard birds were severe, required hospitalization, and one died.
Summary of U.S. human cases with no known animal exposure
The exposure source is unknown for three cases, two cases in children in California and one adult case in Missouri.
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Source: US Centers for Disease Control and Prevention, https://www.cdc.gov/pandemic-flu/php/monitoring/irat-virus-summaries.html
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