Results of Influenza Risk Assessment Tool

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.

Influenza A viruses assessed

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Footnote: In this dot graph, the average score estimates for the potential emergence and public health impact risk elements for the HPAI A(H5N1) clade 2.3.4.4b virus A/California/147/2024 (shown as D) and A/Washington/239/2024 (shown as E) viruses were plotted along with the scores of the previous A(H5N1) clade 2.3.4.4b viruses scored using the IRAT (Figure). The estimates for the current A(H5N1) clade 2.3.4.4b viruses were in the mid-moderate range for both risk of potential emergence and risk of potential public health impact. Additionally, the range of the mean low acceptable and the mean high acceptable scores of A/California/147/2024 and A/Washington/239/2024 viruses overlap with the previously scored A(H5N1) clade 2.3.4.4b viruses (Figure). More information about how the IRAT calculates risk scores is available at Influenza Risk Assessment Tool (IRAT).

Note: The letters in the 'dot' column of this table correspond to those in the dot graph above.

IRAT data table

Virus comparison by risk element score

Footnote: This bar graph shows how the March 14, 2025, IRAT assessments of A/California/147/2024 and A/Washington/239/2024 viruses compares with the June 2024 assessment of A(H5N1) Clade 2.3.4.4b [A/Texas/37/2024] virus and the April 2023 and March 2022 assessments for the A(H5N1) Clade 2.3.4.4b [A/mink/Spain/3691-8_22VIR10586-10/2022] and A(H5N1) Clade 2.3.4.4b [A/American wigeon/South Carolina/AH0195145/2021] viruses, across each of the IRAT's 10 risk elements. These 10 risk elements are weighted differently when scoring a virus' risk of emergence versus its risk of public health impact. Definitions for these risk elements and additional information on how these risk elements are weighted when determining viruses' risk of emergence versus risk of public health are available at Influenza Risk Assessment Tool (IRAT).