Could bird flu become a pandemic? CDC assesses the risk

TThe Centers for Disease Control and Prevention on Friday released a new risk assessment of the H5N1 bird flu virus circulating among dairy cows, slightly increasing its estimate of the virus’s likelihood of causing a pandemic.

The new assessmentdeveloped using the CDC’s Influenza Risk Assessment Tool (IRAT), estimated the risk that the virus could ever cause a pandemic at 5.79, up from a previous score of 5.12 based on an assessment of a related virus in April 2023. Both figures fall within what the CDC tools call a “moderate” risk of 4.0 to 7.9. Some swine flu viruses and the H7N9 bird flu virus score higher than this version of H5N1 according to the IRAT process.

The CDC’s explanation of the new assessment noted that the uncertainty surrounding the scores for the different elements assessed — things like the severity of the illness caused by the viruses and the extent to which people have some immune protection — overlap, which the difference between them is minimal.

Vivien Dugan, director of the CDC’s influenza division, cautioned that the IRAT is a tool for government planning purposes and is not intended to assess risk to the public. The agency continues to characterize the risk posed by H5N1 to the general public as low.

“The tool was actually developed to find a standardized, systematic, statistical way to work with subject matter experts to 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 on pandemic preparedness priorities.

“The tool… tries to answer two questions. What is the risk or probability that a particular virus that is not widely circulating among humans will become a long-term virus transmitted from person to person? That’s the attendance score. And if that virus emerges… what would be the impact on public health?” she said.

The specific virus analyzed in the new assessment came from the first human H5N1 case in the US this year, a farm worker in Texas whose infection was reported in early April. The most recent previous H5N1 risk assessment was based on the analysis of a virus found in 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 fractionally lowered the estimate of the public health impact of a pandemic caused by this virus. The calculations were done based on data up to June 26, before a surge of human cases was discovered among Colorado workers culling chickens at two large H5N1-infected poultry farms. So far, there have been 13 confirmed cases in the US, all with mild infections. Ten of those cases were discovered in Colorado.

“This updated assessment indicates that this is so [Texas virus] scored slightly lower on some risk elements and slightly higher on others compared to 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 influenza viruses using this tool for about a decade. None so far the analyses led to the conclusion that a particular virus posed a high risk of a pandemic.

Recently, the United Kingdom’s Health Security Agency increased its policy pandemic risk assessment of the specific H5N1 virus circulating in cows, from four to three on the six-point scale, in light of “at least five months of ongoing transmission in cattle in the US, with additional mammalian species affected and onward transmission to poultry.”

Translating the IRAT figures into information that is meaningful to individuals is a challenge. So far, none of the viruses analyzed with it have caused a pandemic.

Michael Osterholm, director of the University of Minnesota’s Center for Infectious Diseases Research and Policy, said the current situation with continued transmission of H5N1 in dairy farms is “a conundrum” for flu researchers, given that the virus has a very wide range to infectious diseases. of animals, which recently caused serious diseases in many cases, but in few cases in humans. And the human cases that have occurred in recent years with this clade of H5N1 have been mostly mild.

‘I think the IRAT is trying to put a figure on it [value] to something that none of us can ignore,” Osterholm said.

He noted that despite continued spread among cows, the virus does not appear to have acquired the ability to better attach to the type of receptors found on cells in the upper respiratory tract of humans. If the virus were to acquire that ability, it is believed it could spread more easily to and among humans.

“It seems very difficult to understand all this. We have seen no evidence of an increase [human] binding to the receptor site. And so I think it remains confusing at best,” Osterholm said.