The Centers for Disease Control and Prevention issued a new risk assessment of the H5N1 bird flu virus circulating in dairy cows on Friday, increasing slightly its estimate of the chance it poses of triggering a pandemic.
The new assessment, developed using the CDC’s influenza risk assessment tool or IRAT, gauged the risk the virus might someday cause a pandemic at 5.79, up from a previous score of 5.12 from an assessment of a related virus conducted in April 2023. Both numbers are within what the CDC tool terms a “moderate” risk of 4.0 to 7.9. Some swine influenza viruses and the H7N9 bird flu virus have scored higher than this version of H5N1 using the IRAT process.
The CDC’s explanation of the new assessment noted that the uncertainty around the scores for the various elements that were assessed — things like the severity of disease caused by the viruses and the degree to which humans have any immune protection to them — overlap, making the difference between them minimal.
Vivien Dugan, director of the CDC’s influenza division, cautioned that the IRAT is a tool for government planning purposes, and isn’t meant to gauge the risk for the public. The agency still characterizes the risk H5N1 poses to the general public as low.
“The tool was really developed to find a standardized, systematic, statistical way with subject matter experts to kind of quantify what we think about the future of where a particular virus may go,” she told STAT in an interview. The information is used to inform government decision-making about pandemic preparedness priorities.
“The tool … tries to get an answer to two questions. What’s the risk or the chance that any particular virus that’s not circulating broadly in people will become a sustained human-to-human transmitting virus? That’s the emergence score. And then if that virus emerges … what would be the impact on public health?” she said.
The specific virus analyzed in the new assessment was taken from the first human H5N1 case in the U.S. this year, a farmworker in Texas whose infection was reported at the beginning of April. The most recent previous H5N1 risk assessment was based on analysis of a virus retrieved from an outbreak of H5N1 in mink in Spain in 2023. Both viruses belong to the subset or clade of H5N1 viruses known as 2.3.4.4b.
The new assessment lowered fractionally the estimate of what the public health impact of a pandemic caused by this virus would be. The calculations were run based on data to June 26, before a spate of human cases were detected among workers in Colorado who were culling chickens on two large H5N1-infected poultry operations. As of now, there have been 13 confirmed cases in the U.S., all involving mild infections. Ten of those cases have been detected in Colorado.
“This updated assessment indicates that this [Texas 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,” the new IRAT report said.
The CDC has been analyzing the pandemic potential of non-human flu viruses using this tool for about a decade. To date none of the analyses have led to an assessment that a particular virus constituted a high risk of a pandemic.
Recently the United Kingdom’s Health Security Agency increased its assessment of the pandemic risk of the specific H5N1 virus circulating in cows to four from three on its six-point scale, in light of “at least 5 months of sustained transmission in cattle in the US with additional mammalian species affected and onwards transmission to poultry.”
Translating the IRAT numbers into information that is meaningful to individuals is challenging. To date none of the viruses analyzed using it have gone on to trigger a pandemic.
Michael Osterholm, director of the University of Minnesota’s Center for Infectious Diseases Research and Policy, said the current situation with ongoing transmission of H5N1 in dairy herds is “an enigma” to influenza researchers, given the fact that the virus is infecting a very wide array of animals, causing severe illness in many, but few human cases of late. And the human cases that have occurred in recent years with this clade of H5N1 have been mainly mild.
“I think the IRAT is trying to put a numeric [value] to something that none of us can get our arms around,” Osterholm said.
He noted that despite the ongoing spread in cows, the virus hasn’t yet appeared to have gained the capacity to better attach to the type of receptors that are found on cells in the upper airways of people. If the virus acquired that ability, it is believed it would be able to more easily spread to and among humans.
“To try to make sense of all of this I think is really difficult. We’ve seen no evidence of increased [human] receptor site binding. And so I think it remains at best confusing,” Osterholm said.
