ÐÇ¿ÕÓéÀÖ¹ÙÍø

As of February 28, 2025

ÐÇ¿ÕÓéÀÖ¹ÙÍø assessed the risk from H5N1 viruses to the U.S. general population and to populations in the United States with contact with potentially infected animals, including through contaminated surfaces or fluids. Risk describes the potential public health implications and significance of an outbreak for populations assessed. See definitions below for more detail.

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The purpose of this assessment is to evaluate the current public health risk of H5N1 viruses to the general U.S. population and to those in contact with potentially infected animals or contaminated surfaces or fluids, and to inform public health preparedness efforts.

ÐÇ¿ÕÓéÀÖ¹ÙÍø assessed risk by considering both likelihood and impact of infection in each population (see Methods section and definitions). Both the likelihood and impact of infection are assessed at a population level. Likelihood of infection refers to the probability of infection occurring in the population of interest and considers factors such as the likelihood of transmission to or within the population, the number of people exposed and/or infected, population immunity against infection, and effectiveness or capacity of public health measures to limit spread. Impact of infection considers factors such as the severity of disease, level of population immunity against severe disease, availability of resources to limit impact, and necessary public health response resources.

This assessment outlines the current risk posed by H5N1 viruses to populations in the United States based on currently available data; however, this risk could change. H5N1 viruses are of public health concern because of their pandemic potential. If an H5N1 virus acquires the ability through genetic mutation or reassortment to cause sustained human-to-human transmission, it could cause a pandemic. Because influenza viruses constantly change, ÐÇ¿ÕÓéÀÖ¹ÙÍø monitors these viruses routinely, works to prevent further spread of H5N1 viruses between animals and people, and coordinates H5N1 preparedness activities. ÐÇ¿ÕÓéÀÖ¹ÙÍø will update this risk assessment as needed.

Risk assessment for general U.S. population

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Risk

ÐÇ¿ÕÓéÀÖ¹ÙÍø assesses the overall risk to the general U.S. population as low, with moderate confidence. Of note, for any individual in this population, risk will vary depending on nature of, frequency, and time spent in contact with infected or potentially infected animals or contaminated surfaces or fluids. We also include a section below on factors that could change our assessment.

Likelihood

ÐÇ¿ÕÓéÀÖ¹ÙÍø assesses the likelihood of infection for the general U.S. population as very low. Factors that informed our assessment of likelihood include the following:

To date, there has been no evidence of human-to-human spread of H5N1 viruses in the United States, and there have been very few reported cases among people in the general population to date. The majority of confirmed human H5N1 cases in the United States since 2024 (67 of 70) were associated with exposure to infected animals, including poultry and dairy cows. Three U.S. cases in humans with no known exposure to infected animals have been identified.

• The likelihood of exposure is higher among people in settings where infected birds or dairy cows have been identified, including occupational settings. See our risk assessment for populations in contact with potentially infected animals below.

To date, there has been little evidence of genetic changes that suggest adaptation to humans.

• Genetic analysis of samples from the fatal Louisiana case detected low frequency changes that may result in the increased ability of these viruses to infect the human upper respiratory tract. These or similar changes or mutations may be needed for H5N1 viruses to be able to spread more easily to and among humans.
  • The observed genetic changes in the patient's H5N1 virus, when compared with the virus identified from the patient's backyard poultry (the presumed source of human infection), suggest that the changes were likely generated by virus replication in this patient after hospital admission for advanced disease and were not present at the time of infection.

• Genetic analysis of samples from the Wyoming and Nevada cases found mutations that have previously been associated with more efficient H5N1 virus replication in mammalian cells and in people.

The likelihood of exposure is higher among people consuming raw (unpasteurized) milk from infected animals, although the chance of people acquiring H5N1 virus infection from consuming raw milk is not clear at this time.

• Consumption of raw milk can lead to other serious health outcomes, especially for certain populations. ÐÇ¿ÕÓéÀÖ¹ÙÍø and recommend against consuming raw milk.

• Raw milk sold commercially in California virus. While USDA's tests milk samples from across the country before pasteurization, raw milk in many states can be purchased directly from the farms where it is produced, and may not be captured in these testing protocols.

• There are no confirmed cases of human H5N1 virus infection associated with consuming contaminated raw milk. However, animals such as and have been infected following consumption of milk contaminated with H5N1 virus, and the possibility of human infection with H5N1 virus through ingestion of raw milk cannot be ruled out.

The United States has resources to detect symptomatic human cases of H5N1 and can implement measures to reduce opportunities for onward spread, at the current rate of infection (on average, approximately six to seven cases per month since April 2024).

• Human cases of H5N1 are nationally notifiable, meaning every identified case is investigated by local and state public health and reported to ÐÇ¿ÕÓéÀÖ¹ÙÍø.

• ÐÇ¿ÕÓéÀÖ¹ÙÍø and a wide range of public health and healthcare partners conduct regular monitoring for influenza viruses and illness activity, reviewing data from case reporting, public health laboratory monitoring, clinical laboratory trends, ED visits, and wastewater surveillance.
  • As of February 2025, more than 136,000 specimens have been tested using a protocol that would have detected A(H5) virus, and more than 15,000 people have been monitored after exposure to animals infected with H5N1 virus.
  • ÐÇ¿ÕÓéÀÖ¹ÙÍø now recommends subtyping of all influenza A virus-positive specimens from hospitalized patients on an accelerated basis.
  • ÐÇ¿ÕÓéÀÖ¹ÙÍø has also provided interim recommendations for prevention, monitoring, and public health investigations that indicate, where feasible, an expansion of testing to include an offer of testing to asymptomatic individuals with high likelihood of unprotected exposure to H5N1 virus.
  • Three commercial testing laboratories (Quest Diagnostics, Labcorp, and ARUP) in the United States now offer A(H5) testing, significantly increasing testing access and diagnostic capacity.

Impact

ÐÇ¿ÕÓéÀÖ¹ÙÍø assesses the impact of infection for the general U.S. population as moderate. Factors that informed the assessment of impact include the following:

The majority of reported U.S. cases have had mild illness characterized by conjunctivitis and/or upper respiratory symptoms, irrespective of the genotype. Of the three confirmed cases of H5N1 in humans in 2025, one was in a dairy worker, and two people had exposure to poultry and experienced severe illness requiring hospitalization.

• On December 13, 2024, ÐÇ¿ÕÓéÀÖ¹ÙÍø confirmed a severe case of H5N1 in an individual in Louisiana. The patient was infected with a genotype of H5N1 virus closely related to viruses recently detected in wild birds and poultry in the United States and in recent human cases in British Columbia, Canada, Iowa, and Washington State. The patient in Louisiana died, and the patient in British Columbia was critically ill.

• Historically, there has been a wide clinical spectrum of H5N1 illness (mild to severe), and deaths have occurred. Prior to the current U.S. outbreak, most reported human H5N1 cases since 1997 were identified late in the course of illness after hospital admission and with severe disease (e.g., pneumonia).

The general population is not known to have specific immunity against H5N1 virus.

Medical countermeasures are available to help limit the severity of disease should a member of the general U.S. population become infected.

• Genetic analysis suggests that that H5N1 viruses currently circulating among wild birds, poultry, and dairy cattle in the United States are susceptible to available FDA-approved influenza antiviral medications. Antiviral treatment is currently recommended for patients with confirmed or suspected H5N1 virus infection.

• Antiviral post-exposure prophylaxis can be used to help prevent infection or illness and could be used specifically in those who had unprotected exposure to infected animals.

• Additionally, prompt treatment has been shown to attenuate severe seasonal influenza disease.

• No FDA-authorized or approved vaccines for prevention of H5N1 virus infection are currently commercially available for the general population in the United States.
  • However, under the National Pre-Pandemic Influenza Vaccine Stockpile (NPIVS) program, the Department of Health and Human Services routinely develops vaccines against a wide range of novel influenza A viruses, including H5N1 viruses, and to accelerate the availability of a well-matched vaccine and increase inventory.

The animal and public health response to outbreaks in poultry and dairy cows has societal and economic impacts for the U.S. general population, including the a decrease in and .

Confidence

ÐÇ¿ÕÓéÀÖ¹ÙÍø has moderate confidence in this assessment. This degree of uncertainty is due to several factors, including variability in levels of testing among different animal populations and by geography, as well as the role of wild bird exposure in causing human infections, as the prevalence of H5N1 virus infections in wild birds is difficult to assess. There is additional uncertainty on likelihood of human exposures from other infected animals, including exposure to infected cats (house cats or big cats in zoo and animal sanctuary settings). ÐÇ¿ÕÓéÀÖ¹ÙÍø also recognizes uncertainty in impact, as the effects of transmission route and virus genotype in human infection are unclear.

Risk to populations in contact with potentially infected animals or contaminated surfaces or fluids

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Risk

The risk posed by H5N1 viruses to humans in contact with potentially infected animals or contaminated surfaces or fluids is moderate to high. Of note, for any individual in this population, risk will vary depending on use of workplace controls including personal protective equipment (PPE), nature and frequency of exposure, and time spent in contact with infected or potentially infected animals or contaminated surfaces or fluids. We also include a section below on factors that could change our assessment.

Likelihood

ÐÇ¿ÕÓéÀÖ¹ÙÍø assesses the likelihood of infection for populations in contact with potentially infected animals or contaminated surfaces or fluids as low to high. Individual risk will vary depending on the frequency, duration, and nature of contact with potentially infected animals. The likelihood of exposure from important classes of animals are outlined below:

The likelihood of exposure to H5N1 viruses from infected commercial poultry and dairy cows is moderate to high.

• H5N1 viruses are widespread in U.S. poultry and dairy cows, with detections in more than 160,000,000 birds in all 50 states since January 2022 and more than dairy herds in 17 states since January 2024.

• The majority of individuals with confirmed H5N1 virus infection (67 of 70) had exposures associated with commercial agriculture or backyard poultry.
  • Of these, 41 were exposed to dairy herds (cows) and 24 were exposed to poultry farms and culling operations.
  • Some workers were identified as cases before any animals on the farm had tested positive for H5N1.
  • Among dairy workers (including veterinarians), the likelihood of exposure may be particularly high among those working in milking parlors and other environments with contaminated surfaces and fluids.
  • Use of PPE is low among this population, and increased availability and use can decrease the likelihood of exposure, especially for workers in direct contact with animals or their secretions.
  • Several serosurveys have been conducted to identify recent infections that may not have been detected. For example, in a serosurvey of bovine practitioners, evidence of infection was found in three individuals who were asymptomatic, including two without exposures to animals with known or suspected H5N1 virus infection and one who did not practice in a state with known infected cattle. In a serosurvey of dairy workers in Michigan and Colorado, 7% of those tested had serologic evidence of infection.

The likelihood of exposure to H5N1 viruses from infected non-commercial poultry and wild birds is moderate.

• H5N1 viruses also circulate among non-commercial poultry and wild birds, and among some other wild animals. H5N1 viruses have been detected in more than 12,000 wild birds across 52 jurisdictions in the United States since January 2022.
  • Exposure to sick or dying birds infected with H5N1 viruses, including potentially through contaminated fluids or surfaces, raises the likelihood of human infection. Exposure risk may be elevated in populations with animal contact, such as backyard poultry farmers and wild bird or waterfowl hunters.

The likelihood of exposure to H5N1 viruses from recreational animal activities, such as visiting agricultural fairs, livestock shows, or petting zoos, is low to moderate.

• None of the reported cases had a known exposure to an animal involved in a recreational animal activity.

Animal monitoring and infection control measures have the potential to reduce the likelihood of exposure to H5N1 viruses for people.

• require mandatory testing of lactating dairy cows prior to interstate movement, and require herd owners with positive cows to provide epidemiological information to facilitate contact tracing and disease monitoring. Additionally, national testing programs are in place for and poultry.

• Extensive monitoring of exposed persons and public health control efforts are in place. ÐÇ¿ÕÓéÀÖ¹ÙÍø has interim recommendations for prevention, monitoring, and public health investigations of human cases of H5N1. ÐÇ¿ÕÓéÀÖ¹ÙÍø also has recommendations for worker protection and use of personal protective equipment (PPE) to reduce the risk of exposure.

• People with job-related or recreational exposures to birds or infected mammals can reduce their chance of infection using appropriate precautions to protect against H5N1.

Impact

ÐÇ¿ÕÓéÀÖ¹ÙÍø assesses the impact of human infection via exposure to potentially infected animals as moderate. In addition to the factors outlined in the Impact section for the general population, ÐÇ¿ÕÓéÀÖ¹ÙÍø notes economic and policy impacts, including the financial loss associated with population culling, product disposal, and loss of production. As of November 2024, more than has been spent on response to ongoing H5N1 outbreaks in animals, and

Confidence

ÐÇ¿ÕÓéÀÖ¹ÙÍø has moderate confidence in this assessment. This degree of uncertainty is due to factors including variability in levels of A(H5) testing among different animal populations and by geography, as well as the role of wild bird exposure in human infection, as the true prevalence of infection in animal populations is difficult to assess. There is additional uncertainty on likelihood of exposure to other infected animals, including exposure to infected cats (house cats or big cats in zoo and animal sanctuary settings). ÐÇ¿ÕÓéÀÖ¹ÙÍø also notes uncertainty in impact due to limited evidence on whether illness severity differs by transmission route and virus genotype.

Future Risk

This assessment outlines the risk posed to the United States by H5N1 viruses as of February 28, 2025, but ÐÇ¿ÕÓéÀÖ¹ÙÍø stresses this risk could change, as influenza A viruses can mutate quickly, and therefore have the potential to cause pandemics.

The viral changes needed to cause a pandemic are unpredictable. However, the high prevalence of H5N1 virus infections among animals in close contact with humans increases opportunities for mutation or reassortment that could lead to sustained person-to-person spread, causing a pandemic. It is possible that co-infections with seasonal influenza A and H5N1 viruses in the same person or animal provide opportunities for reassortment of genes between two influenza A viruses, potentially resulting in an influenza A virus with characteristics of both seasonal influenza A and H5N1 viruses that is more efficiently transmitted among people than current H5N1 viruses circulating among birds, cows, and other animals.

H5N1 virus infection can cause severe illness in people; H5N1 viruses historically have caused severe cases in humans. ÐÇ¿ÕÓéÀÖ¹ÙÍø has developed H5 candidate vaccine viruses (CVVs) that are expected to be effective against H5N1 viruses now circulating among wild birds, poultry, and cows in the United States. These H5 CVVs could be used to produce a vaccine for people, if needed, thereby reducing the risk for severe disease among humans. Access to antivirals for treatment or post-exposure prophylaxis could also decrease future risk of severe illness or transmission.

Factors that could change our assessment

ÐÇ¿ÕÓéÀÖ¹ÙÍø continues to monitor for additional factors that could change the risk assessment, including:

Background and Methods

Definitions