Updating the CDC's IRAT (Influenza Risk Assessment Tool) Rankings

Credit NIAID

#12,575



Last September, before H7N9 reinvented itself (twice) in China, we looked at the CDC's ranking (see CDC: IRAT Evaluation Of Novel Avian & Swine Flu Risks) of 11 novel flu subtypes/strains that circulate in non-human hosts and are believed to possess some degree of pandemic potential.

The CDC is quick to point out the Influenza Risk Assessment Tool (IRAT) is not meant to be predictive.  As stated in their FAQ:

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.

But the IRAT can help planners decide which viruses pose the greatest risks, so they can prioritize their efforts and investments. And ten months ago, out of the 11 viruses on the list:

• 5 were listed as having a moderate Potential Emergence Risk.
• 5 were listed as having a moderate Potential Impact Risk
• Only 1 (H7N9) was listed as having High-Moderate Potential Impact Risk 

Not all novel flu viruses are created equally.  Some show signs of being more likely to jump to humans than others, while some have shown more virulence in humans than others.   

A mild flu, even if it spreads easily, isn't that much of a threat to public health. But a severe flu that spreads easily, is another matter entirely. 

The CDC uses two sets of criteria to evaluate novel viruses.  One to estimate a virus's potential for sustained human-to-human transmission, and another to guage it's potential for significant impact on public health. 

Fast forward to June 2017, and the CDC has added two new viruses to their IRAT list (a second H7N9 [A/Hong Kong/125/2017] variant, and the recently arrived Canine H3N2) and has further refined their rankings.   

Where there were 11 last September, there are now 13. It is worth nothing that of the 13 novel viruses on this list, 10 have been added in the past 6 years. 

Below you'll find the latest rankings, with the new H7N9 virus nudging out the old version in both Emergence and Impact scores, putting both (A & B) far ahead of the pack.

Summaries on the two new entries, follow:

H3N2: [A/canine/Illinois/12191/2015]
The H3N2 canine influenza virus is an avian flu virus that adapted to infect dogs. This virus is different from human seasonal H3N2 viruses. Canine influenza A H3N2 virus was first detected in dogs in South Korea in 2007 and has since been reported in China and Thailand. It was first detected in dogs in the United States in April 2015. H3N2 canine influenza has reportedly infected some cats as well as dogs. There have been no reports of human cases.

Summary:  The average summary risk score for the virus to achieve sustained human-to-human transmission was low risk (less than 4). The average summary risk score for the virus to significantly impact public health if it were to achieve sustained human-to-human transmission was in the low risk range (less than 4).

Some past blogs on the Canine H3N2 virus include:

A Canine H3N2 Virus With PA Gene From Avian H9N2 - Korea

Canine H3N2 Reassortant With pH1N1 Matrix Gene

Virology J: Human-like H3N2 Influenza Viruses In Dogs - Guangxi, China

The bigger concern comes from the recently emerged Yangtze River Delta lineage of H7N9 (we don't have enough data yet on the risks posed by the recently emerged HPAI version).

H7N9: [A/Hong Kong/125/2017]
Low pathogenic avian influenza (LPAI) H7N9 viruses were first reported from China in March 2013. These viruses were first scored using the IRAT in March 2013 and again in April 2013, and then annually in 2014, 2015, and 2016 with no change in overall risk scores. Between October 2016 and May 2017 evidence of two divergent lineages of these viruses was detected – the Pearl River Delta lineage and the Yangtze River Delta lineage. The IRAT was used to assess LPAI H7N9 [A/Hong Kong/125/2017], a representative of the Yangtze River Delta viruses.

Summary:  A risk assessment of H7N9 [A/Hong Kong/125/2017] was conducted in May 2017. The overall IRAT risk assessment score for this virus falls into the moderate-high risk category and is similar to the scores for the previous H7N9 viruses. The summary average risk score for the virus to achieve sustained human-to-human transmission is in the moderate risk category (less than 7). The summary average risk score for the virus to significantly impact public health if it were to achieve sustained human-to-human transmission was in the moderate-high risk category (less than 8).

While IRAT can't tell us which virus will spark the next pandemic, or when that might happen, there are three major takeaways from these reports.

1. There are a lot of novel flu threats out there with at least some pandemic potential. IRAT currently evalluates 13, but that is by no means an exhaustive list.
2. New viruses appear to be emerging at an accelerated rate over the past decade.
3. Viral evolution requires that these viruses be re-evaluated from time to time, as nothing is static in the flu world. A virus that ranks relatively low today might be viewed as a serious contender a year or two from now.

All very good reasons why pandemic preparedess is getting renewed attention by the CDC, the WHO, and other government and NGO agencies around the world.  You'll find some recent blogs on that below:

HHS Pandemic Influenza Plan - 2017 Update

OpenWHO: Preparedness Training For Epidemics, Pandemics & Health Emergencies

World Bank: World Ill-Prepared For A Pandemic

Are We Prepared to Help Low-Resource Populations Mitigate a Severe Pandemic?

Community Pandemic Mitigation's Primary Goal : Flattening The Curve