03/19/2025 | Press release | Distributed by Public on 03/19/2025 10:58
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.)
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:
Historically, the mortality rate from avian influenza observed globally has been around 50%; however, only one of 70 human infections* in the United States to date has resulted in death. Recent studies have reported that ferrets previously infected with seasonal influenza A(H1N1) virus had less severe illness from H5N1 bird flu. While more study is needed, pre-existing antibodies could contribute to decreasing the severity of H5 bird flu illness in U.S. cases.
A CDC study published on February 21, 2025, in Emerging Infectious Diseases found that ferrets previously infected with seasonal influenza A(H1N1)pdm09 virus had developed cross-reactive antibodies to some components of an avian influenza A(H5N1) virus. When these ferrets were later exposed to an avian influenza A(H5N1) virus, they exhibited reduced viral replication and decreased onward spread of avian influenza A(H5N1) virus compared with ferrets that had not been previously infected with A(H1N1)pdm09 virus and did not have these cross-reactive antibodies. Overall, these findings in ferrets suggested that prior seasonal influenza virus infection with an A(H1N1)pdm09 virus may provide some level of protection against clade 2.3.4.4b avian influenza A(H5N1) viruses.
A second study, published on March 17, 2025, in The Lancet Microbe reported similar findings. Ferrets with antibodies from previous infection with the seasonal influenza A(H1N1)pdm09 virus [A/California/7/2009] that were later infected with avian influenza A(H5N1) virus had less severe illness and were less likely to spread the virus to other ferrets in the same enclosure compared to ferrets with no preexisting immunity to influenza virus. This study only looked at prior infection and did not look at the effects of prior vaccination in ferrets, so it's not possible to draw conclusions from this study on the potential effect seasonal flu vaccines might have on reducing severity of H5N1 bird flu illness in ferrets or in people; seasonal influenza vaccines are not designed or intended to prevent H5N1 bird flu disease. The study also found that when ferrets were exposed to an avian influenza A(H5N1) clade 2.3.4.4b virus [A/Texas/37/2024] via the surface of their eyes, they developed severe and transmissible disease just as they did after respiratory exposure, highlighting the importance of following recommendations for eye protection for people with exposure to animals infected or potentially infected with avian influenza A(H5N1) viruses.
As part of the study above, researchers also tested blood specimens from healthy adults who had previously received two doses of a pre-pandemic clade 2.3.4.4b bird flu vaccine (adjuvanted A/gyrfalcon/WA/410886/2014/H5N8) from the National pre-pandemic Influenza Vaccine Stockpile. Results showed that 57% of people who received both doses of pre-pandemic vaccine developed (i) neutralizing antibodies at levels associated with immune protection and (ii) HA binding antibodies that were cross-reactive to the clade 2.3.4.4b avian influenza A(H5N1) viruses that are causing the current outbreak in dairy cattle and poultry with sporadic infections in people. These findings suggest that stockpiled pre-pandemic vaccines can provide protection against the avian influenza A(H5N1) viruses currently circulating in animals in the United States.
Another recent report, published on March 7, 2025, in Emerging Infectious Diseases assessed the immune responses of two dairy farm workers in Michigan who tested positive for H5N1 bird flu following work related exposure to infected dairy cows. One of the two workers, who reported having mild illness with symptoms like eye redness (conjunctivitis) had an immune response resulting in the development of neutralizing antibodies against avian influenza A(H5N1) virus. Clade 2.3.4.4b avian influenza A(H5N1) virus was isolated from this person. The other person did not develop neutralizing antibodies. This is the first study conducted in people to assess immunity to clinically mild illness from A(H5) virus infection. Prior to this study, limited data were available on immune responses to H5N1 bird flu among people with clinically mild illness like conjunctivitis.
CDC regularly performs sequencing of seasonal influenza A and B viruses and novel influenza A viruses, including A(H5N1) viruses, to assess for genetic changes known to be associated with antiviral resistance. In a new CDC study published in Emerging Infectious Diseases on March 7, 2025, CDC scientists assessed the antiviral susceptibility of clade 2.3.2.1c A(H5N1) viruses and clade 2.3.4.4b A(H5N1) viruses collected from humans in Cambodia, United States, and Chile. The study found that except for two viruses isolated from humans in Cambodia, all viruses were susceptible to M2 ion channel-blockers in cell culture-based assays. All viruses were susceptible to the PA cap-dependent endonuclease inhibitor class of antiviral drugs, baloxavir and tivoxavir, and to the polymerase basic 2 (PB2) inhibitor antiviral drug, pimodivir. All viruses also displayed susceptibility to neuraminidase inhibitor class of antiviral drugs, which includes oseltamivir, zanamivir, peramivir, laninamivir, and AV5080. 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. Flu antiviral drugs, including oseltamivir, work best when started as soon as possible, ideally within two days after flu symptoms begin.
* One additional case was previously detected in Colorado in a poultry worker who experienced mild illness in 2022.