At a glance
The Influenza Risk Assessment Tool (IRAT) is an evaluation tool conceived by CDC and further developed with assistance from global animal and human health influenza experts. The IRAT is used to assess the potential pandemic risk posed by influenza A viruses that are not currently circulating in people.
Purpose
The current overall individual and population health risk to the general public posed by the avian influenza A(H5N1) virus presently spreading in cows, poultry, and other mammals remains low. Systematic comparisons of data related to this avian influenza A(H5N1) virus using the Influenza Risk Assessment Tool (IRAT) to data from other influenza A viruses has scored this virus's future pandemic potential as "moderate risk" based on information through June 26, 2024. This is similar to previous assessments of earlier avian influenza A(H5N1) viruses.
The IRAT uses expert opinion to evaluate the potential of a representative novel influenza virus to gain the ability for person-to-person spread and the resulting potential public health impact if that were to happen, compared to other viruses evaluated in past IRAT reports. 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.
This report summarizes the findings of an IRAT conducted on a recent avian influenza A(H5N1) virus from a human case in Texas (A/Texas/37/2024). The score places the currently circulating avian influenza A(H5N1) virus 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 June 26, 2024, prior to additional human cases in Colorado. The report was completed expeditiously based on data as of June 26 to inform ongoing preparedness discussions.
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 August 9, 2024, CDC published a new IRAT assessment for A/Texas/37/2024, an avian influenza A(H5N1) clade 2.3.4.4b virus that was associated with a human infection in a person in Texas who had exposure to A(H5N1) virus-infected dairy cows. Avian influenza A(H5N1) clade 2.3.4.4b viruses are causing outbreaks in wild birds and poultry, occasional infections in mammals, and rare human infections in the United States and globally. Between April 1, 2024 and August 9, 2024, eight human cases of A(H5N1) bird flu and five human cases of A(H5) bird flu were reported in the United States. The first of these was reported on April 1, 2024, when the State of Texas announced that a person tested positive for HPAI A(H5N1) virus.
Previously, CDC had assessed two other avian influenza A(H5N1) clade 2.3.4.4b viruses: the A/American wigeon/South Carolina/AH0195145/2021 and the A/mink/Spain/3691-8_22VIR10586-10/2022. Both viruses had overall estimated IRAT scores in the moderate risk category range of 4.0 to 7.9.
This updated assessment includes new information since then, including information from additional human cases reported in the United States (but excluding recent human cases among poultry workers associated with exposure to infected poultry at poultry farms in Colorado). This updated assessment using the virus from the first human case in Texas, indicates that this A/Texas/37/2024 virus has 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." New data visualizations have been added to this page to aid in the comparison of these viruses.
The Influenza Risk Assessment Tool (IRAT) is an evaluation tool conceived by CDC and further developed with assistance from global animal and human health influenza experts. The IRAT is used to assess the potential pandemic risk posed by influenza A viruses that are not currently circulating in people. The IRAT does not predict pandemics, and the IRAT is not used to assess the risk of H5N1 bird flu to the general public, which current remains low. 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 viruses with pandemic potential emerge or undergo a change in characteristics that prompts the need for a new assessment.
Background
The IRAT is a tool used to assess the potential pandemic risk posed by influenza A viruses. It is designed for use by public health professionals for pandemic preparedness purposes. It is not intended to be used to determine the current public health risk to the general public. The current risk to the general public from contemporary H5N1 avian influenza (bird flu) viruses is still considered low. The current H5N1 bird flu continues to be mostly an animal health issue. Human infections with H5N1 bird flu are rare, and these viruses are not well-adapted for spreading among people, as they do not currently have the ability to easily infect the human upper respiratory tract. Most past human infections have occurred following close, unprotected contact with sick or dead birds.
Influenza A viruses assessed
Footnote: This dot graph plots the average weighted risk scores for "public health impact" and "emergence" for viruses subjectively assessed by the IRAT's panel of expert reviewers. As shown on the graph, virus "A/Texas", is in the mid-moderate range for both risk of potential emergence and risk of potential public health impact. The average score estimates ranked this virus sixth for the emergence and seventh for the impact risks when compared to the other 15 viruses scored with the IRAT to date. 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.
Virus comparison by risk element score
Footnote: This bar graph shows how the June 2024 assessment of A(H5N1) Clade 2.3.4.4b [A/Texas/37/2024] virus compares to 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).