What Is Avian Flu?

Introduction

First discovered in 1878, avian influenza is a viral disease that primarily infects birds, although some strains can spread to and kill mammals, including humans. The H5N1 strain, one of the most common today, was first identified in domestic waterfowl in 1996.

Birds. The flu mainly spreads asymptomatically among wild aquatic birds, but domestic poultry are also susceptible and can become extremely ill and die. Twenty-five known subtypes of avian flu have been identified in birds. There are two main categories: low pathogenicity, which often causes little-to-no signs of disease, and high pathogenicity, strains that cause severe disease and poultry mortality. Highly pathogenic strains of avian influenza have a 90 to 100 percent mortality rate among poultry. Most infected livestock die within forty-eight hours.

Humans. Human cases of bird flu are extremely rare, but often deadly. Most people who become infected have usually been directly exposed to infected poultry or their environments. Human-to-human transmission is believed to have only occurred in a few isolated instances, but sustained transmission between humans has not yet been identified. The two strains of avian flu that have killed the most people are H5N1 and H7N9. The first human cases of H5N1 were detected in China in 1997, but the bulk of cases came between 2003 and 2015, with Egypt as a hotspot. To date, there have been at least 904 human cases, with 464 deaths across nearly two dozen countries, according to World Health Organization (WHO) data. The United States saw a rise of H5N1 cases in late 2024, spreading to at least sixty-five people by year’s end. The first U.S. death linked to avian flu occurred in January 2025 in a patient with other underlying health conditions.

H7N9 was also first reported in China in 2013, and it has killed more than six hundred people to date. Some non-fatal cases are asymptomatic, but common effects can include flu-like symptoms, conjunctivitis (pink eye), pneumonia, multi-organ failure, seizures, and neurological damage. Newer strains that later evolved, such as H5N6 and H5N8, are also infecting humans. 

Other mammals. Since 2020, avian flu has been found in close to fifty mammal species across more than two dozen countries, making this wave considerably more widespread than previous ones. The disease can often be deadly for mammals. By January 2024, avian flu had reportedly begun worsening in marine mammals at rates that prompted scientists to become concerned about mammal-to-mammal transmission, which would represent a much more dangerous threat to the affected ecosystems. In April, the virus was detected in U.S. cattle for the first time, infiltrating herds in several states. By December 2024, California declared a state of emergency over the disease’s rapid spread to about three-quarters of U.S. dairy herds. Because of the prevalence in cattle, most of the humans that were infected during the 2024 United States outbreak are dairy farmers, and experts believe there are far more human cases going undetected in this group.

U.S. health officials are also watchful of the pork industry. “What’s also concerning and what will precipitate greater spread is if the virus were to find itself into the pig population,” Nirav Shah, principal deputy director of the U.S. Centers for Disease Control and Prevention (CDC), said at a CFR event in May. “[Pigs] are, for various biological reasons, perfect vessels through which an even more virulent strain could emerge.”

Where is avian flu?

H5N1 began circulating in parts of Africa, Asia, and Europe in late 2021, but its reach has exploded since then: the virus has been recorded in more than eighty countries since 2022. There have been recent cases in all continents except for Australia.

In the current wave, relentless outbreaks in poultry and cattle have wracked U.S. agriculture nationwide as well. Avian flu has been recorded in forty-eight states and affected more than eighty-two million poultry, as of March 2024. European countries witnessed a similar pattern of surging infections, affecting some fifty million poultry across thirty-seven countries, though France suffered the brunt of these. Other countries, such as Japan and South Africa, experienced their worst avian flu outbreaks among farmed birds.

What are the economic consequences of the outbreaks?

Avian flu outbreaks can cost farmers and consumers billions of dollars in lost profits and higher prices, and they disrupt important international trade. The ongoing avian flu outbreak in the United States—the world’s largest poultry meat producer and second-largest egg producer— caused economic losses around $2.5–3 billion by the end of 2023, and experts predicted this cost would only rise in new surges of the disease. Avian flu has led to the deaths or culling of more than 125 million poultry livestock. Per U.S. regulations, producers must cull all the birds in an infected flock, even if some of them do not have the disease. Amid the outbreak, the cost of a dozen eggs more than doubled to nearly five dollars in January 2023 before producers were able to restore their flocks. In recently infected cattle, U.S. farmers have reported reduced milk production, although this has had a limited effect on supplies thus far. The American Association of Bovine Producers has tallied the economic hit to farmers to be around $100–200 per cow. 

The reduction of poultry has exacerbated food security in some parts of the world, particularly in lower-income countries where poultry products are a major source of protein. South Africa’s poultry industry underwent major supply shortages in 2023 as its worst avian flu outbreak added to its chronic power outages, which further hampered poultry production. Studies in Egypt have recorded stunted growth in youth coinciding with past avian flu outbreaks.

Avian flu and the perceptions around it can also harm the global trade of poultry products. Amid a 2004 outbreak in the United States, for example, U.S. poultry exports dipped more than 20 percent after several countries imposed embargoes on U.S. poultry products. These trade constrictions also reduce incomes for producers whose flocks never even contracted the virus, as well as for others involved in the poultry value chain. These economic concerns have rendered several countries wary of vaccinating poultry for avian flu, as they do not want to risk potential embargoes from their trading partners.

What is driving the spread of avian flu?

The global spread is due in part to bird migration patterns, allowing infected birds to carry the disease thousands of miles to sites along their flyways. At the same time, the virus is increasingly mutating as it encounters new and more host animals and species.

Outbreaks in the past often subsided in the summer when bird migrations ceased, yet this was not the case in the recent H5N1 strain, which was some of the cause for scientists’ concern. The constant stream of outbreaks across the world in 2023 and 2024 triggered concerns that avian flu could become a year-round threat. The reasoning for the virus’s persistence is still not fully clear, although some research has indicated that climate change and human activities are contributing factors. Climate change-related fluctuation in conditions such as temperature are altering the timing of some bird behaviors, including migration and breeding. Meanwhile, habitat destruction and urbanization are also affecting breeding site availability. The confluence of these factors drives birds from different parts of the world to come into contact with each other for the first time and interact with more habitats, both natural and urban. Such encounters expand the opportunities for the virus to spread and mutate. 

The corporate consolidation of U.S. factory farms, where livestock are typically held in close quarters, also contributes to the rapid spread of the virus and mass poultry deaths. The environmental nonprofit Food and Water Watch said in a 2020 study [PDF] that 96 percent of U.S. chickens are raised under a production contract by major corporations, such as Tyson Foods or Perdue Farms.   

How significant is the threat to public health?

Human cases of avian flu are still relatively rare, and experts say the threat to public health remains generally low. However, human cases have historically had a high fatality rate, at more than 50 percent. (At its peak, for comparison, the global COVID-19 death rate was reported to be around 8 percent, though it is widely thought to be far higher.) Still, health experts are worried about the mounting risks to agricultural workers who could be regularly exposed to infected animals. New research shows that pregnant women are especially at risk of mortality, too.

For a virus to have pandemic potential, it needs to be able to pass easily from person-to-person. H5N1 does not yet have the ability to do so, but scientists fear it could develop this feature. A study published in December 2024 found that the current version circulating in cattle has the ability to bind to human receptors after only a single mutation, increasing the risk that it could become a pandemic.

“The bigger picture is that this virus is not cooling off,” former CFR Senior Fellow Jennifer Nuzzo told Politico. “We’ve been worrying about this virus for more than twenty years. And in the last year, it has really been remarkable in how far across the globe it has been spreading, and how many species it’s been affecting.”

Is there a vaccine for avian flu?

There are avian flu vaccines for humans and birds, but there are a number of challenges and uncertainties with both. For instance, there is global hesitancy over the bird vaccine for trade reasons, and the existing human vaccines are derived from previous strains, so it could take a while to develop new ones and implement them on a mass scale. Some countries fear that importing vaccinated birds will make conducting surveillance more difficult, as it will be harder to determine if birds have been infected or vaccinated. To some degree, there are concerns that importing eggs or meat from birds that have been vaccinated could inadvertently spread the disease within the importing country’s borders, though there is no evidence to suggest this is possible. 

Avian flu vaccines are mostly administered to birds in countries where the flu is endemic and that have minimal poultry trade, such as Egypt, Indonesia, Mexico, and most recently, France. Commercial poultry in the United States receive other vaccinations, such as against fowlpox, but not yet for avian flu. The U.S. Department of Agriculture is testing several bird vaccines but has not approved any, mostly over concerns that requiring a vaccine will harm trade. Despite this roadblock, experts have increasingly called for the U.S. government to launch a vaccination campaign for poultry, which they say would reduce the risk of spillover that could trigger a pandemic. Some poultry vaccines have proven to be as high as 94 percent effective. 

There is also a lot of uncertainty around the prospects of an avian flu vaccine for humans. Several major companies that manufacture them have said they could expedite the production of millions of doses in the event of a pandemic. The United States has an emergency stockpile of H5N1 vaccines, but experts have said that they would be insufficient if this particular strain were to start infecting people at a larger scale. Further, the shots have only been administered in trials and were derived from strains that circulated twenty years ago.

Some companies are working on developing vaccines that better match the highly pathogenic subvariant that is causing havoc today, but experts say it is unclear whether a human vaccine could be manufactured at a global scale while staying ahead of the virus’s rapid mutation. “The virus is always changing,” the CDC’s Shah told CFR in May 2024. “If we made 300 million doses, the virus could shift in a month from now, as we’ve seen with COVID. So we have to be careful about oversupplying because we may be producing something that is soon out of date.”

CFR’s Luciana Borio agrees, noting that the current U.S. stockpile is not a guarantee that will guard against today’s strains. “We shouldn’t be overconfident in our ability to make vaccines that are safe and effective for avian flu,” Borio adds. “Clinical trials involving H5N1 in the past have yielded lackluster results.”  

And as with many diseases, there is a global equity issue: these vaccines, which are not even past the testing phase, are already slated to go to richer countries first, leaving lower-income countries at risk. “We can’t do a repeat of what happened with COVID, where countries that were vaccine-producing waited until there was no more interest in vaccines in their populations before they really worked to consider the needs of developing countries and non-vaccine-producing countries,” Borio says.

In its final days, the Biden administration nearly doubled U.S. government funding for avian flu, but said they would not yet authorize the stockpiled vaccines for use, leaving that decision to the incoming administration of Donald Trump.