H5 Avian influenza response: Focus areas for ongoing assessment of public health risk | Bird flu

Current H5N1 bird flu risk

People who are at increased risk include:

• Farmers and workers who work with infected animals or their by-products
• Owners of flock of birds in the backyard
• Animal care workers (eg vets, wildlife workers)
• Animal health and public health responders.

But what factors would influence a change in the CDC’s current risk assessment to the public? The following is a description of the epidemiologic and virologic characteristics of the avian influenza situation that CDC researchers are tracking to formulate the agency’s immediate avian influenza risk assessment and further calibrate the avian influenza response to protect public health:

• Virus transmission: How do viruses spread and how efficiently is it spread?
• Severity of the disease: How sick do people get with H5N1 bird flu infections?
• Case allocation: 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 the effectiveness of antiviral drugs and vaccines?

Virus transmission

What is the CDC looking for? Persistent human-to-human transmission outside a household increases the likelihood of a significant impact on public health.

Influenza A(H5N1) has been spread 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 mink, 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 bird flu has occasionally been reported globally. To date, there is no evidence of human-to-human transmission associated with the current bird flu situation in the United States. Transmission identified outside a household will be of greater concern than within a household when assessing the immediate risk to public health.

In addition to watching for human-to-human transmission through case investigations, CDC continues to rapidly analyze and share genetic sequences of samples from human cases, and alongside information obtained from viral samples from infected animals, CDC monitors for changes that will possible for the virus to spread more easily – especially to humans and other mammals.

The severity of the disease

What is the CDC looking for? The CDC is concerned about all people who become infected with bird flu and is especially concerned if we begin to see people who rapidly become seriously ill and require hospitalization, or who die from the infection. Severe illness may indicate that the virus has changed and is now better able to make people seriously ill. This severity can have a greater impact on public health, burden the health system and can have other societal and economic consequences (eg 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 one reported in Canada and one reported in Louisiana, resulted in serious illness. The severity of the illness can be affected by a number of factors, including acquired genetic changes to the virus, the amount of virus to which the infected person was exposed, the route of infection, underlying health conditions, how long the person was ill and the timeliness of medical care/treatment, or a combination of all these factors.

Case allocation

What is the CDC looking for? Indication that this virus may have wide spread among humans within specific populations or to the general population, or increasing numbers of humans becoming infected without apparent exposure to infected animals.

Human cases associated with the ongoing outbreak have been sporadic and almost all have followed identifiable exposures to dairy cows, poultry and/or other animals.

Wide spread of cases would be evident if all of the following were to happen:

• Numerous sporadic (ie occurring at irregular intervals or rarely as isolated events) human cases not related to expected shared/joint animal exposures
• Cases that occur in multiple geographic locations
• Cases that occur close together in time

Effects of genetic changes in the virus

What is the CDC looking 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 routinely assesses the sequences of human and animal viruses for changes that may affect infectivity or transmissibility in humans, the accuracy of diagnostic tests, and the effectiveness of vaccines or antiviral agents. 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 affect cross-protection if A(H5) vaccines were required for use in humans. Also, no changes have been identified in the receptor binding domain of the virus except for low-frequency changes in the fatal case from Louisiana and the severe case from Canada. These changes were thought to have occurred after the individuals had been infected rather than being acquired from their infectious exposure. There is no evidence that viruses with these changes spread beyond these patients.

Taken together, these data indicate that A(H5N1) viruses circulating in animals retain avian receptor binding properties without significant changes that would affect infectivity or transmissibility in humans. In addition, 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 affect the performance of H5 influenza diagnostic tests used for testing in all US states and at the CDC.

These factors are all important considerations that inform what public health measures should be implemented in the H5 avian influenza public health response. If we see changes in these factors, additional actions may be necessary to protect the health and safety of people with potential animal exposure 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 bird flu, such as who should receive pre- or post-exposure prophylaxis, testing strategy and how best to use personal protective equipment.
• Procuring additional treatments and vaccines to ensure we have an adequate supply for those who would benefit from their use.
• Initiation of a voluntary H5 vaccination program focused on people with foreseeable exposure to the virus.
• Initiation of a wider voluntary H5 vaccination program if the possibility of widespread transmission or increasing disease severity is identified.

Such an escalation is likely to require additional resources.