CDC’s Influenza Risk Assessment Tool (IRAT)

Summary

What CDC knows

An IRAT is used to evaluate the future pandemic potential of an influenza A virus currently circulating in animals. Experts review data and score a representative flu virus based on the likelihood that the virus may gain the ability to spread easily from person-to-person. Experts also score the virus on the impact that it could have on public health. IRAT scores may be updated when there are important changes to virus characteristics, like the ability to infect a new kind of animal or human.

What CDC is doing

CDC issued a new IRAT report on a virus from the current U.S. 2024 H5N1 bird flu outbreak which indicates the virus has a moderate future potential pandemic risk when compared to other Influenza A viruses. This information is used to guide preparedness planning. CDC believes that the current risk to the public's health from this virus is low. CDC continues to work with USDA, FDA, and state health departments to monitor and test people who have been exposed to infected or potentially infected animals.

The IRAT is used for evaluation of flu viruses

The Influenza Risk Assessment Tool (IRAT) was created by CDC and U.S. government experts for use in flu pandemic preparedness. The IRAT seeks expert opinion to score several characteristics of a representative influenza A virus. Together these scores indicate the likelihood that a virus will gain the ability to spread from person-to-person and the potential public health impact if that were to happen. The IRAT is used to plan for, not to predict, the next pandemic. It does not calculate the immediate or current risk to the general public. CDC uses this tool as a part of its ongoing public health work. This analysis, which includes data through June 26, did not include the 10 human infections that occurred in Colorado in July following exposure to infected poultry (9 cases) and dairy cows (1 case). All were associated with mild illness and a majority experienced conjunctivitis.

The IRAT seeks to answer two questions about a flu virus

An IRAT evaluation is done when a non-human flu virus that can spread to people appears or has changed in some way. The IRAT seeks to answer the following two questions:

• Emergence: What is the risk that the flu virus will emerge to spread from person-to-person?
• Public health impact: If that were to happen, what would the impact be on public health?

The IRAT considers multiple risk elements to measure potential pandemic risk

There are three overarching categories with 10 risk elements that measure the potential flu pandemic risk. These are:

1. The properties of the virus, including genomic analysis, type of host and tissues and cells that the virus is best matched to, the ability of the virus to spread efficiently in animal studies, and the predicted effectiveness of available antiviral medications.
2. Population attributes, including whether there is existing immune protection in people, the severity of illness caused by the virus in people and animals, and how similar the virus in question is to currently available seasonal flu vaccines, candidate vaccine viruses, or stockpiled pre-pandemic vaccines.
3. Ecologic and Epidemiologic factors, including how widespread the virus is in animals, the kinds of animals that are impacted, the likelihood of human contact, and the frequency of human infections.

CDC and other U.S. government flu experts score these elements to estimate potential pandemic risk. A score is given for potential emergence risk, potential impact risk, and also an overall summary score. The scores fall within one of three groups:

• Low risk (point score between 1.0 – 3.9);
• Moderate risk (point score between 4 – 7.9);
• High risk (point score between 8.0 – 10.0).

Because these scores can involve a degree of subjectivity, flu experts provide a range of acceptable scores for each risk element. Then each risk element is weighted based on its importance in answering the questions about emergence and public health impact. From there, the scores are ranked and compared to the potential pandemic risk of other influenza A viruses. The IRAT is designed to be flexible and responsive, so sometimes the rank of risk elements may change, other risk elements can be added, and others can be dropped.

The IRAT is not a predictive tool

The IRAT does have some limitations. It often relies on data that are scarce or preliminary. Also, it is subjective and relies heavily on expert opinion. It's important to keep in mind that the IRAT does not measure the current risk to the public's health. It focuses on the perceived potential of a flu virus to become a pandemic and the impact that would result if that were to happen. Additionally, the IRAT does not predict future pandemics.

The current risk to the general public's health remains low

In response to the ongoing U.S. 2024 multistate H5N1 outbreak, characterized by the emergence of H5N1 in a new host (cows), CDC conducted a new IRAT assessment. The most recent IRAT was done on the H5N1 virus isolated from a person in Texas. CDC then wrote up a report summarizing the results. CDC has done several of these assessments around H5 bird flu viruses over the years, as the agency has tracked the emergence and evolution of this group of viruses. The latest IRAT indicates that the overall future potential pandemic risk of H5N1 virus remains moderate and supports the ongoing, coordinated U.S. government response. The current risk to individual and population health remains low. This score has not significantly changed from the most recent assessment done on a similar H5N1 virus that caused an outbreak among mink in Spain in 2023.