Over the past decade we've seen an explosion in the number of novel flu viruses (avian and swine) with zoonotic potential that can (and have) infected humans. Prior to 2009, H5N1 was considered the biggest novel flu threat, followed far behind by H9N2 and some H7 viruses which had produced mostly mild illness in a handful of people.
Since then we've seen the emergence of H7N9 (2 lineages + LPAI & HPAI), H5N6, H10N8, H1N2, H3N2v, H1N1v, H6N1, H7N2, H7N3 along with some contenders for future zoonotic transmission, including avian H5N8 and canine H3N2.In order to keep track, and evaluate these threats the CDC developed their IRAT (Influenza Risk Assessment Tool) which currently follows 14 novel flu subtypes/strains that circulate in non-human hosts and are believed to possess some degree of pandemic potential.
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 gauge it's potential for significant impact on public health.
Over the past few year's we've looked at this system (see here and here), particularly after new subtypes or strains have been added. Over the last 18 months, the IRAT has added 3 new viruses (Canine H3N2, a second lineage of H7N9, and North American H7N8).
For a much more detailed explanation of how IRAT works, we've the following research article from the CDC's EID Journal.
Volume 24, Number 3—March 2018(Continue . . . )
Use of Influenza Risk Assessment Tool for Prepandemic Preparedness
Stephen A. BurkeComments to Author and Susan C. Trock
Author affiliations: Centers for Disease Control and Prevention, Atlanta, Georgia, USA (S.C. Trock, S.A. Burke); Battelle, Atlanta (S.A. Burke)
In 2010, the Centers for Disease Control and Prevention began to develop an Influenza Risk Assessment Tool (IRAT) to methodically capture and assess information relating to influenza A viruses not currently circulating among humans. The IRAT uses a multiattribute, additive model to generate a summary risk score for each virus. Although the IRAT is not intended to predict the next pandemic influenza A virus, it has provided input into prepandemic preparedness decisions.
Planning and preparation for influenza pandemics are major challenges to public health authorities for many reasons, not the least of which is the inherent variability and unpredictability of the influenza virus (1). Just in the past decade, infections from multiple new influenza viruses have occurred in humans, representing influenza A subtypes such as H1N2, H3N2v, H5N1, H5N6, H6N1, H7N2, H7N3, H7N7, H7N9, H9N2, and H10N8. In response to these findings, prepandemic vaccines were developed for some of these viruses (2–5). In 2009, a new virus, subsequently designated influenza A(H1N1)pdm09, emerged in humans in North America and quickly spread, causing the first influenza pandemic of the 21st century (6). Although only 3 hemagglutinin (HA) subtypes of influenza (H1, H2, and H3) are known to have caused human pandemics (7), the emergence and spread of influenza A(H5N1) and, more recently influenza A(H7N9), with associated high death rates in humans, are of great concern. If these or other influenza A viruses not currently circulating among humans develop the capability to transmit efficiently among humans, they pose a risk for causing a pandemic that could be associated with high rates of illness and death (8,9).
The task of risk mitigation planning and preparedness for pandemic influenza is difficult, and a tool is needed that systematically evaluates different influenza viruses to inform decisions related to the prioritization and allocation of resources for vaccine development, influenza surveillance strategies, and research initiatives. In this context, the Centers for Disease Control and Prevention (CDC; Atlanta, GA, USA) developed the Influenza Risk Assessment Tool (IRAT) with the goal to systematically evaluate influenza A viruses that are not circulating in humans but potentially pose a pandemic risk (10).
The IRAT uses a common decision analysis approach that incorporates input from multiple elements or attributes, applies a weighting scheme, and generates a score to compare various options or decisions (11). In regard to the evaluation of animal-origin influenza viruses for their potential human pandemic risk, 2 specific questions were developed related to the potential risk for emergence and consequent potential impact: 1) What is the risk that a virus not currently circulating in humans has the potential for sustained human-to-human transmission? (emergence question); and 2) If a virus were to achieve sustained human-to-human transmission, what is the risk that a virus not currently circulating among humans has the potential for substantial impact on public health? (impact question).