CDC Study Raises Concerns Over Undetected H5N1 Infections in Humans

Experts and heads of health agencies continue to voice concerns over the endemic H5N1 avian influenza infecting animals and humans across the United States. There have been 68 confirmed human cases of H5N1 reported by the CDC as of February 14, 2025; however, a recent study published by the agency suggests that this number is an undercount because of asymptomatic illness.^1,2

Gloved hand holding blood test for H5N1 avian influenza | Image Credit: © angellodeco - stock.adobe.com

"It means our surveillance is inadequate," Lauren Sauer, a professor at the University of Nebraska Medical Center College of Public Health, told NPR. "Any detection of asymptomatic or mild cases in this study just tells me we're missing cases."³

H5N1, although typically spread amongst animals, can be passed to humans through exposure to infected animals or animal carcasses. Infection symptoms can range from mild to severe respiratory symptoms and conjunctivitis, and some have resulted in hospitalizations, as well as one death in Louisiana. In the last quarter of 2024, case reports of the Louisiana patient and another younger patient in British Columbia raised alarm among health care professionals and researchers who found mutations in the patients’ viruses. These mutations made it easier for viral particles to attach to respiratory receptors in the lung, potentially enabling and supporting human-to-human transmission. However, there have been no reports of this as of February 14, 2025.⁴

In the study released by the CDC, experts found evidence that H5N1 is spread silently among bovine veterinarians, who have frequent, prolonged exposure to animals. They evaluated 150 veterinarians across 46 sites using a questionnaire coupled with a voluntary blood test to test for antibodies to a recent HPAI A(H5) virus infection. Among the participants, 55% practiced in states with virus-positive dairy herds, and 17% worked with dairy cattle with known or suspected infection.¹

Of the surveyed patients, 2% had antibodies to recent HPAI A(H5) infection and reported no respiratory symptoms or conjunctivitis. According to the CDC report, the 3 veterinarians were exposed to multiple animals, including dairy and non-dairy cattle, at sites with no known or suspected viral infection, signaling that the virus is going undetected in cattle populations as well. Additionally, one of the veterinarians worked with virus-positive poultry.¹

As of quarter 1 of 2025, there have been 2 more hospitalizations due to serious illness. In Wyoming, a woman was hospitalized after contracting H5N1 bird flu from her backyard flock. The CDC reported she had preexisting conditions, underscoring the seriousness of H5N1 infection in vulnerable populations. Another case was reported of an Ohio farmer who was hospitalized after exposure to infected poultry and has since been discharged.^5,6

The H5N1 virus strains found in the Ohio and Wyoming cases were a less severe strain called B3.13; however, a new strain called D1.1 has begun to dominate among bird and dairy cattle populations. This was the confirmed strain behind the fatality of the patient from Louisiana.⁶

The primary concern remains animal-to-human transmission of H5N1, and although human infections remain rare, the virus is spreading rapidly among commercial, backyard, and wild birds, prompting the culling of infected flocks. The virus has also been detected in other animal species, such as dairy cattle and even domestic cats. A report out of Oregon triggered an urgent warning about raw pet food after 2 indoor cats contracted H5N1 bird flu and had to be euthanized. This has not only raised concerns about the safety of raw pet food, but also the potential for animal-to-human transmission via domesticated, indoor pets without exposure to infected animals.^4,7

"We should never underestimate flu," says Sauer. "If cases are occurring more frequently than detected in humans, we risk missing small changes that allow the virus to begin to spread much more easily in humans."³

H5N1 bird flu vaccine vial | Image Credit: © catalin - stock.adobe.com

Efforts are being made to monitor, control, and respond to the situation as it unfolds. On February 18, 2025, the US Department of Agriculture gave conditional approval for an updated flu vaccine to protect poultry against H5N1, which has affected over 150 million commercial and backyard flocks. Additionally, the former Biden Administration and the US Department of Health and Human Services granted Moderna $590 million to accelerate H5N1 mRNA vaccine development.^8,9

The continued spread of H5N1 among animals, alongside mounting evidence of undetected human infections, underscores the urgent need for stronger surveillance and preparedness. The discovery of asymptomatic cases in veterinarians raises concerns that current monitoring efforts may be missing key transmission pathways, particularly as new strains like D1.1 emerge. Improving surveillance and early detection will be essential in understanding the virus’s trajectory and guiding effective response efforts.

REFERENCES

1. Notes from the field: Seroprevalence of highly pathogenic avian influenza A(H5) virus infections among bovine veterinary practitioners — United States, September 2024. CDC. February 13, 2025. Accessed February 18, 2025. https://www.cdc.gov/mmwr/volumes/74/wr/mm7404a2.htm

2. H5 bird flu: Current situation. CDC. February 14, 2025. Accessed February 18, 2025. https://www.cdc.gov/bird-flu/situation-summary/index.html

3. After delay, CDC releases data signaling bird flu spread undetected in cows and people. NPR. February 13, 2025. Accessed February 18, 2025. https://www.npr.org/sections/shots-health-news/2025/02/13/nx-s1-5296672/cdc-bird-flu-study-mmwr-veterinarians

4. As human cases of H5N1 rise, experts voice concerns about mutations. Pharmacy Times. January 6, 2025. Accessed February 18, 2025. https://www.pharmacytimes.com/view/as-human-cases-of-h5n1-rise-experts-voice-concerns-about-mutations

5. Wyoming woman hospitalized with bird flu. NBC News. February 17, 2025. Accessed February 18, 2025. https://www.nbcnews.com/health/health-news/bird-flu-hospitalized-woman-chickens-wyoming-rcna192455

6. U.S. bird flu hospitalizations rise to 4 after Ohio discloses case. CBS News. February 15, 2025. Accessed February 18, 2025. https://www.cbsnews.com/news/us-bird-flu-hospitalizations-rise-ohio-discloses-case/

7. Bird flu cases in household cats spark raw pet food warning. Newsweek. February 16, 2025. Accessed February 18, 2025. https://www.newsweek.com/bird-flu-cases-household-cat-raw-pet-food-warning-2031986

8. Updated bird flu vaccine for poultry gets license. Axios. February 18, 2025. Accessed February 18, 2025. https://www.axios.com/2025/02/18/updated-bird-flu-vaccine-licensed

9. US Department of Health and Human Services grants $590 million to accelerate H5N1 mRNA vaccine development.I. January 20, 2025. Accessed February 18, 2025. https://www.pharmacytimes.com/view/us-department-of-health-and-human-services-grants-590-million-to-accelerate-h5n1-mrna-vaccine-development