CDC A(H5N1) Bird Flu Response Update March 19, 2025

#18,381

Today the CDC released their first bird flu response update since late February which recaps several recent studies we've previously looked at (see here, here, and here), along with providing new information on serological testing of a child in California, and genetic sequencing of a human infection from Ohio. 

Three days ago John Korslund's blog  announced that a new genotype - D1.3 - was behind the recent surge in H5N1 outbreaks in the upper Midwest (Ohio & Indiana) and was likely strain involved in a recent human case in Ohio.

Today the CDC confirms that information, identifying the Ohio case as stemming from the clade 2.3.4.4b virus of the D1.3 genotype.  Over the past 12 months we've seen 4 new genotypes of note emerge in the United States.

• B3.13 aka the `bovine' strain affecting dairy cattle in at least 16 states and mildly infecting dozens of humans
• D1.1   a wild bird/poultry strain which has spilled over into > a dozen people in Washington State, severely infected a teenager in British Columbia, and produced a fatal infection in Louisiana. 
• D1.2  a wild bird/poultry strain which recently detected in poultry and 2 pigs in Oregon
• D1.3  a recently detected wild bird/poultry strain which has been infection poultry and at least 1 human

That said, more than 100 genotypes of H5N1 have been reported in North America since the H5 virus arrived in late 2021, and more are expected to emerge over time.

First some excepts from today's update (follow the link to read the report in its entirety), after which I'll have a brief postscript. 

 

CDC A(H5N1) Bird Flu Response Update March 19, 2025

At a glance

In this spotlight, CDC provides laboratory updates on serology testing from close contacts of a confirmed case of influenza A(H5) virus infection in a child in California, as well as sequencing information for A(H5) viruses related to the most recent Ohio human case. CDC also summarizes findings from recently published studies.

Risk assessment

CDC recently published an updated avian influenza A(H5N1) virus risk assessment: The current risk to the general U.S. population is low. The risk to populations exposed to potentially avian influenza A(H5N1) virus-infected animals, including through contaminated surfaces or fluids, is currently assessed as moderate to high. CDC has moderate confidence in this assessment. (Moderate confidence, in this context, means that the assessment is based on credibly sourced and plausible information, but it acknowledges some information gaps or assumptions that underlie the analysis.)

Background

CDC has been responding to the public health challenge posed by a multistate outbreak of avian influenza A(H5N1) virus, or "H5N1 bird flu," in dairy cows, poultry, and other animals in the United States since the spring of 2024. CDC has been working in collaboration with the U.S. Department of Agriculture (USDA), the Food and Drug Administration (FDA), the Administration for Strategic Preparedness and Response (ASPR), state public health and animal health officials, and other partners using a One Health approach.

Since April 2024, 70 human cases of avian influenza A(H5) virus infection have been reported in the United States. Of these, 41 cases were associated with exposure to sick dairy cows and 26 were associated with exposure to avian influenza A(H5N1) virus-infected poultry. The source of the exposure in 3 cases, could not be determined. To date, human-to-human transmission of influenza A(H5) virus has not been identified in the United States. The immediate risk to the general public from H5 bird flu remains low.

On the animal health side, USDA is reporting that since March 2024, 989 dairy herds in 17 U.S. states have confirmed cases of avian influenza A(H5N1) virus infections in dairy cows as the number of affected herds continues to grow. USDA reports that since April 2024, there have been avian influenza A(H5) virus detections in 336 commercial flocks and 207 backyard flocks, for a total of more than 90.9 million birds affected.

Among other activities previously reported in past spotlights and ongoing, recent highlights of CDC's response to this include:

Recent updates

Laboratory

CDC completed serology testing on blood specimens from close contacts of a child with mild illness in San Francisco who was confirmed to be positive for avian influenza A(H5N1) virus, though, there were no known animal exposures associated with that case. Serology testing was conducted to look for antibodies to influenza A(H5N1) virus in this child, which would indicate recent infection. The child's blood was tested and found to have antibodies to avian influenza A(H5N1) virus. None of the close contacts of the case in San Francisco who were tested had antibodies to avian influenza A(H5N1) virus, which supports the conclusion that none of these close contacts were infected, and that no person-to-person spread occurred among these close contacts. These findings are reassuring. To date, human-to-human transmission of influenza A(H5) virus has not been identified in the United States.

CDC has sequenced the virus from the most recent Ohio human case. Genetic data have been posted in GISAID (Epi ID 19785793) and have been submitted to GenBank. Sequencing indicates this is a clade 2.3.4.4b virus of the D1.3 genotype based on classification using USDA's genotyping assignment criteria. D1.3 viruses, like D1.1 viruses, originated from A3 genotype A(H5N1) viruses that were introduced to North America in 2022 and have subsequently reassorted with North American wild bird avian influenza viruses. There were no markers that would impact the effectiveness of influenza antivirals or existing candidate vaccine viruses. Finally, CDC did not identify changes that would make this virus better adapted to spread among or infect mammals. Attempts to isolate this virus in eggs are ongoing.

          (Continue . . . )

There are obviously a number of studies published each month on H5N1, and since each looks at different aspects of the virus, they don't all come to the same conclusions.   

In this update the CDC discusses the findings of a recent EID Journal report (Antiviral Susceptibility of Influenza A(H5N1) Clade 2.3.2.1c and 2.3.4.4b Viruses from Humans, 2023–2024), which we looked at 2 weeks ago.

Their analysis acknowledges a 10-fold reduction in NA inhibition by oseltamivir against clade 2.3.4.4b viruses, but describe this is unlikely to have a significant impact. 

Oseltamivir was approximately 10-fold less active at inhibiting the neuraminidase activity of clade 2.3.4.4b viruses and approximately 3-fold less active against clade 2.3.2.1c viruses, when compared to seasonal influenza A viruses. The clinical significance of these laboratory findings, however, is unknown.

Significant reduction in antiviral susceptibility is considered to be greater than 100-fold reduction. The laboratory findings in this study, therefore, indicate that these A(H5N1) viruses are likely to retain susceptibility to oseltamivir. Additionally, these findings do not support changing the current recommendations for antiviral treatment of human infections with novel influenza A viruses, including A(H5). CDC continues to recommend prompt treatment with oseltamivir for people with confirmed or suspected A(H5N1) virus infection.

Other recent studies we've seen have been less reassuring. 

St. Jude Researchers: Current Antivirals Likely Less Effective Against Severe Infection Caused by Bird Flu in Cows’ Milk

Emerg. Microbes & Inf: Oseltamivir Resistant H5N1 (Genotype D1.1) found On 8 Canadian Poultry Farms

Even if we assume that oseltamivir will retain most of its NA inhibiting effect, getting it to those who need it early in the course of their infection will be a challenge (see 2022's CDC HAN #0482: Prioritizing Antiviral Treatment of Influenza in the Setting of Reduced Availability of Oseltamivir).

Any way you cut it, prevention is always preferable to treatment.