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Current H5N1 bird flu risk
People who are at increased risk include:
-- Farmers and workers who work with infected animals or their byproducts
-- Backyard bird flock owners
-- Animal care workers (e.g., veterinarians, wild animal facility workers)
-- Animal health and public health responders.
But what factors would influence a change to CDC’s current risk assessment for the general public? What follows is a description of the epidemiological and virologic characteristics of the avian influenza situation that CDC scientists are tracking to formulate the agency’s immediate avian flu risk assessment and further calibrating the avian flu response to protect the public’s health:
-- Virus transmission: How is virus spreading and how efficiently does it spread?
-- Disease severity: How ill do people with H5N1 bird flu infections become?
-- Case distribution: How widespread are cases?
-- Effects of genetic changes in the virus: What is the impact of genetic changes to the virus on infectivity or transmissibility, the accuracy of diagnostic tests, and effectiveness of antiviral drugs and vaccines?
Virus transmission
What is CDC on the lookout for?
Sustained human-to-human transmission outside of a household increases the likelihood of significant public health impact.
Influenza A(H5N1) has been spreading in wild birds globally since the mid-1990s and in the United States since 2014. The virus initially spread to commercial and backyard poultry and has also infected mammals, including minks, sea lions and now dairy cattle.
There have been sporadic human cases both in the United States and in other countries, and limited human-to-human transmission of avian influenza has been occasionally reported globally.
To date, there is no evidence of human-to-human transmission associated with the current avian influenza situation in the United States.
Transmission identified outside of a household would be of greater concern than within a household when assessing immediate public health risk.
Beyond looking out for human-to-human transmission through case investigation, CDC continues to rapidly analyze and share genetic sequences of samples from human cases and, alongside information gained from viral samples from infected animals, is monitoring for changes that would allow the virus to spread more easily—particularly to humans and other mammals.
Disease severity
What is CDC on the lookout for?
CDC is concerned about all people who become infected with avian flu and is particularly concerned if we begin seeing people who quickly become severely ill and require hospitalization or who die of the infection.
Severe disease may indicate the virus has changed and is now better able to make people severely ill.
This degree of severity could have a greater public health impact, straining the healthcare system and may have other societal and economic impacts (e.g., if people cannot work).
Most cases of H5N1 bird flu associated with the ongoing outbreak in the United States have resulted in mild symptoms.
CDC experts and other scientists continue to work to understand why some infections, including an infection reported in Canada and one reported in Louisiana, resulted in serious illness.
Severity of illness can be impacted by a number of factors, including acquired genetic changes of the virus, the amount of virus to which the infected people were exposed, the route of transmission, underlying health conditions, how long the person was sick and the timeliness of medical care/treatment, or some combination of all these factors.
Case distribution
What is CDC on the lookout for?
Indication that that virus may have broad dissemination among humans within specific populations or to the general population, or increasing numbers of people who are becoming infected without clear exposure to infected animals.
Human cases associated with the ongoing outbreak have been sporadic, and nearly all have followed identifiable exposures to dairy cows, poultry, and/or other animals.
Broad dissemination of cases would be evident if all of the following were to occur:
-- Numerous sporadic (i.e., occurring at irregular intervals or infrequently as isolated events) human cases unrelated to expected shared/common animal exposures
-- Cases occurring in multiple geographic locations
-- Cases occurring close together in time
Effects of genetic changes in the virus
What is CDC on the lookout for?
Genetic changes known to be associated with increased severity or transmissibility or other viral changes seen at the same time as increased transmissibility and increasing severity of infection.
CDC conducts routine assessment of the sequences of the viruses from humans and animals for changes that might impact infectivity or transmissibility in humans, the accuracy of diagnostic tests and the effectiveness of vaccines or antivirals.
To date, genetic analysis has not identified changes in viruses compared to available clade 2.3.4.4b candidate vaccine viruses (CVVs) that would be predicted to impact cross-protection if A(H5) vaccines were needed for use in humans.
Nor have changes been identified in the receptor binding domain of viruses except for low frequency changes in the fatal case from Louisiana and the severe case from Canada.
These changes were believed to have occurred after the individuals were infected rather than acquired from their infecting exposure.
There is no evidence that viruses with these changes spread beyond these patients.
Collectively, these data indicate that A(H5N1) viruses circulating in animals retain avian receptor binding properties with no significant changes that would impact infectivity or transmissibility in humans.
Additionally, there have been only a few sporadic changes identified in viruses detected in animals or humans associated with mammalian adaptation or slightly reduced susceptibility to commercially available antiviral drugs.
Finally, no changes have been identified in viruses that impact the performance of H5 influenza diagnostic tests that are used for testing across all U.S. states and at CDC.
These factors are all important considerations that inform what public health actions should be implemented in the H5 avian flu public health response.
Should we see concerning changes in these factors, additional actions may be necessary to protect the health and safety of people with potential animal exposures as well as the general public.
Additional actions may include but are not limited to:
-- Updating guidance to better protect those who may be exposed to H5 avian flu, such as who should receive pre- or post-exposure prophylaxis, testing strategy, and how to best use personal protective equipment.
-- Procuring additional treatments and vaccines, to ensure we have sufficient supply for those who would benefit from their use.
-- Initiating a voluntary H5 vaccination program focused on people with predictable exposure to the virus.
-- Initiating a broader voluntary H5 vaccination program if the possibility of widespread transmission or increasing disease severity is found.
Such escalation will likely require additional resources.
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Source: US Centers for Disease Control and Prevention, https://www.cdc.gov/bird-flu/spotlights/h5n1-response-01142025.html
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