Seasonal Influenza (Flu)

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Influenza Risk Assessment Tool (IRAT)

Questions & Answers

What is the Influenza Risk Assessment Tool (IRAT)?

The Influenza Risk Assessment Tool (IRAT) is an evaluation tool being developed by CDC and external influenza experts that measures the potential pandemic risk posed by influenza A viruses that currently circulate in animals but not in humans. The IRAT makes an assessment of potential pandemic risk based on two different scenarios: “emergence” and “public health impact.”

“Emergence” refers to the risk of a novel (i.e., new in humans) influenza virus acquiring the ability to spread easily and efficiently in people. “Public health impact” refers to the potential severity of human disease caused by the virus (e.g., deaths and hospitalizations) as well as the burden on society (e.g., missed workdays, strain on hospital capacity and resources, and interruption of basic public services) if a novel influenza virus began spreading efficiently and sustainably among people.

Ten scientific criteria are used to measure the potential pandemic risk associated with each of these scenarios. These 10 criteria can be grouped into three overarching categories: “properties of the virus,” “attributes of the population,” and “ecology & epidemiology of the virus.” Influenza subject matter experts will evaluate novel influenza viruses based on each of these 10 criteria. Each of the 10 criteria is then weighted statistically based on its significance to each of the two scenarios. A composite score for each virus is then calculated based on the given scenario. These composite scores provide a means to rank and compare influenza viruses to each other in terms of their potential pandemic risk for each of the two scenarios.

Can the IRAT predict a future pandemic?

No. The IRAT is an evaluative tool, not a predictive tool. Flu is unpredictable, as are future pandemics.

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What is the purpose of the IRAT?

The IRAT is intended to do the following:

Prioritize and maximize investments in pandemic preparedness by helping to determine which novel (new) influenza viruses to develop vaccines against and capitalizing on surveillance efforts and in-country capacity building activities;
Identify key gaps in information and knowledge which can be the basis to prompt additional studies. (For example, if information is not available for one of the 10 criteria used by the IRAT additional studies could be done or resources allocated to provide the needed information);
Document in a transparent manner the data and scientific process used to inform management decisions associated with pandemic preparedness;
Provide a flexible means to easily and regularly update the risk assessment of novel influenza viruses as new information becomes available;
Be an effective communications tool for policy makers and the influenza community;
Provide a means to weigh the 10 evaluation criteria differently depending on whether the intent of the risk assessment is to measure the ability of an influenza virus to “emerge” as a pandemic capable virus (i.e., become capable of efficient human-to-human spread) or “impact” the human population.

Does the IRAT have any limitations?

Yes. The IRAT cannot predict the next pandemic. Furthermore, the IRAT is not intended to eliminate the need for subject matter expertise. In fact, subject matter experts are needed to carefully analyze the 10 criteria used by the IRAT to make determinations of pandemic risk and to rank the importance of the criteria according to the specific risk question or situation. Lastly, the IRAT is not intended to make exact risk estimates. For example, many risk assessments generate a quantitative measure that describes the likelihood of exposure or disease risk. The IRAT focuses on the perceived pandemic potential of novel influenza viruses as estimated by subject matter experts using the IRAT evaluation criteria and available data.

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What are the evaluation criteria used by the IRAT?

The IRAT consists of 10 evaluation criteria grouped within three overarching categories. These categories and criteria are described as follows:

The “Properties of the Virus” category contains four of the 10 evaluation criteria, including:
1. Genomic variation is a measure of how quickly an influenza virus mutates or changes and the rate at which the influenza virus undergoes “reassortment” (i.e., exchanges genetic material with other influenza viruses).
2. Receptor binding refers to the host preference (e.g., animal or human) of an influenza virus as well as the types of tissues and cells the virus is best suited to infecting (e.g., nose tissue and cells vs. deep lung tissue and cells). Some influenza viruses are better adapted to infecting humans as opposed to animals.
3. Transmission in lab animals is a measure of the ability of an influenza virus to transmit efficiently in animals in laboratory studies. Some influenza viruses can transmit through the air via small infectious droplets expelled through coughs or sneezes, whereas other influenza viruses may only spread through direct contact with an infected host.
4. Antiviral treatment susceptibility/resistance is a measure of how well an influenza virus responds to treatment with influenza antiviral drugs, such as oseltamivir, zanamivir and M2 blockers.
The “Attributes of the Population” category contains three of the 10 evaluation criteria, including:
1. Existing population immunity refers to whether the human population has any existing immune protection against the novel influenza virus being evaluated. Susceptibility to infection and severity of illness associated with specific influenza viruses may depend on age, geographic area, or genetic factors.
2. Disease severity and pathogenesis measures the severity of illness caused by a particular influenza virus in people and/or animals.
3. Antigenic relationship to vaccine candidates is a measure of how similar a novel influenza virus is when compared to a current or previously manufactured influenza vaccine strain.
The “Ecology and Epidemiology” category contains the final three evaluation criteria, including:
1. Global distribution (animals) measures of how widespread an influenza virus is in animals. For example, is the virus found in animals in a limited area or is it found in animals from many different areas?
2. Infection in animal species refers to what kinds of animals are impacted by the influenza virus and the likelihood of human contact with these animals. For example, are influenza infections occurring in wild birds or domestic birds?
3. Human Infections determines whether human infections with a novel influenza virus are occurring. If so, under what circumstances are human infections occurring? For example, has human-to-human transmission or clusters of disease occurred? Alternatively, how frequently and easily does transmission occur after direct and prolonged contact between humans and infected animals?

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How are the IRAT’s 10 evaluation criteria ranked and weighted?

Each of the 10 evaluation criteria provided in the IRAT tool are used by influenza experts to generate point scores estimating the potential pandemic risk associated with that criterion. The point scores fall into three general classifications of risk: low risk, moderate risk and high risk.

“low risk” is associated with a point score between 1 and 3;
“moderate risk” is associated with a point score between 4-7; and,
“high risk” is associated with a point score between 8-10.

Each of the 10 evaluation criteria also are weighted according to importance to each of the two scenarios: emergence and public health impact. As an example, when assessing an influenza virus for “emergence,” the evaluation criteria may be ranked and weighted as follows (with the first criterion receiving the highest rank and weight score, and the last criterion receiving the lowest rank and weight score).

Human infections
Transmission in lab animals
Receptor binding
Existing population immunity
Infection in animal species
Genomic variation
Antigenic relationship to vaccine candidates
Global distribution (animals)
Disease severity and pathogenesis
Antiviral treatment susceptibility/resistance

As an additional example, when assessing an influenza virus for “public health impact,” the evaluation criteria could be ranked and weighted as follows (with the first criterion receiving the highest rank and weight score, and the last criterion receiving the lowest rank and weight score).