WARNING: GRAPHIC IMAGERY
Cows dying from H5N1 left to rot in the open in California, where the deadly virus has spread to hundreds of farms and at least 27 people. Source
On an ordinary spring morning in 1957, American microbiologist Maurice Hilleman opened the New York Times and immediately jumped to a headline few others noticed. Thousands of miles away in Hong Kong, the Times reported, more than 250,000 people had fallen ill with a flu-like illness. His immediate reaction: "My God, this is the pandemic. It's here!"
Pandemics caused by new strains of influenza (more colloquially known as the flu) have a way of hitting without warning, a pattern recognized since the first known flu pandemic in 1510. In 1918, this was particularly obvious when the Spanish flu killed tens of millions worldwide within months of the first known case. Though less lethal, subsequent pandemics in 1957, 1968, and 1977 spread even more rapidly with the advent of air travel. During the 2009 swine flu pandemic, arguably the first such outbreak detected by modern surveillance systems, the first case was still found far too late for containment.
This time, humanity may not be far behind the curve in the next flu pandemic. We may already be tracking it.
Since 1997, H5N1, a strain of influenza primarily found in birds, has slowly emerged as a pandemic threat. Having resulted in nearly 500 reported deaths and 1000 cases, H5N1 is often at the top of government pandemic plans. Its dramatic effects have captured the media's imagination—the New York Times described it as a "doomsday virus," and on Fox News, a former US CDC director called it the next "great pandemic." While these headlines are only speculation, the virus does appear capable of mutating to infect people better and cause lethal disease.
For decades, H5N1 has come and gone as a health concern, but now, some things appear to have changed for the worse. Past H5N1 scares often related to outbreaks in farmed poultry that spilled over into farm workers. Then, in 2020, H5N1 suddenly expanded its host range and sparked a panzootic—a pandemic in animals—after a more infectious strain of the virus emerged. It found new niches among birds and entrenched itself in wild birds, making containment difficult. It also spread to previously unaffected mammalian species. Some sporadically died from the virus in the wild, but a few—seals in South America, mink in Europe—suffered massive die-offs. Notably, the virus wiped out 97% of elephant seal pups in Argentina, and other mass mortality events were suspected in seals from Chile to the Caspian Sea.
In 2023, beaches in Argentina were littered with the corpses of seals struck down by H5N1. Source
Then, sometime in late 2023 or early 2024, the virus jumped into dairy cows in Texas. When cases were first detected in late March of 2024, it was unclear if this was just a lucky infection from birds. However, cases quickly began to pop up in farms from North Carolina to Michigan. The virus also sporadically infected cats, mice, and humans on those farms and spread to other farmed animals, including poultry, goats, and alpacas. Surprisingly, the traditionally bird-adapted virus appeared to survive quite well in this new host, giving it new opportunities to infect humans.
Meanwhile, the response from American authorities can only be described as inadequate. Seven months into the outbreak, experts are still scrambling to answer basic questions about the outbreak. How are cows transmitting the virus to each other? What percent of cows die? Will recovered animals become permanently immune, or could the virus resurge on previously affected farms? How many human infections are being missed by surveillance systems? Much of this uncertainty is directly linked to a plethora of shortfalls in the US response, from limited testing to the slow reporting of genetic information and data.
A damning investigative report in Vanity Fair put a spotlight on the reasons behind the bungled response. From the start, profits came into conflict with public health. Instead of fighting the virus, US government agencies, state officials, and the agricultural industry fought one another. The White House and Center for Disease Control (CDC) pushed for more aggressive measures, but were resisted by the US Department of Agriculture (USDA), which moved to protect farmers concerned about profits and panic. Outbreaks could take months to report, and veterinarians who dared speak up about H5N1 were fired. In Texas, ground zero for the outbreak, state officials refused to stop moving infected animals to other states, leading to a nationwide outbreak.
To comprehend the degree to which response efforts are failing, it is perhaps to hear from those near the frontlines. Paul Friedrichs, a former US army general turned key pandemic response official, described his experience in the current H5N1 outbreak as worse than his combat experience in Iraq. University of Minnesota expert Michael Osterholm stated, "one day we could wake up and have everyone look at the US and say, "If you thought that they had a poor response in Wuhan with COVID, look at what you guys did with H5N1."
On the ground, H5N1 is regaining momentum after a late summer lull in infections. New outbreaks have been reported in cows in Utah and Idaho, but the most dramatic reports have come out of California, the nation's largest dairy producer. Despite a mandatory testing order to detect the virus, the outbreak is not under control. In a matter of months, H5N1 has been confirmed in around 300 farms in the state, mainly affecting the Central Valley region. Consequently, at least 27 dairy workers also contracted the virus. With a dairy industry nearly three times larger than any other affected state, California's outbreak poses an unprecedented risk in terms of both economic damage and disease spread.
H5N1 is rapidly spreading in California. Source
While H5N1 has generally caused mild illness in cows compared to other species that experience nearly 100% death rates, mildness isn't guaranteed. Initially, affected farms saw 2% death rates, though California's cows are experiencing mortality rates of up to 20%—a ten-fold increase. The deaths have overwhelmed local infrastructure in the state, causing dead cows to pile up on the roadside.
There is a silver lining to the California outbreak: even with cases increasing exponentially, the virus hasn't mutated significantly yet. Reassuringly, most human infections have only experienced mild symptoms and eye inflammation, and are linked to animal contact. If increases in symptom severity or cases of human-to-human spread occur, that would significantly increase the risk of a pandemic.
However, arguably more concerning developments have occurred outside of California. In late August, a hospitalized patient in Missouri was tested as part of routine flu surveillance. Though H5N1 was not detected in animals in the state, the patient was confirmed to have the virus, setting off alarm bells. The patient had no contact with sick animals and lacked any other apparent risk factor for catching the virus. Sequencing of the virus from the patient revealed two key mutations, one significantly reducing the effectiveness of vaccines for H5N1. One of the patient's household contacts also fell ill and had antibodies against the virus. Fortunately, no more outbreaks have been detected in Missouri, but the cases could indicate undetected, possibly human-to-human spread.
Simultaneously, an outbreak distinct from the cattle epidemic is occurring along the Pacific Flyway, a route for migratory birds stretching from Alaska to Mexico. With mass migrations underway, infected birds are sparking human and animal infections wherever they land. So far, massive poultry outbreaks in Washington and Oregon have resulted in 15 human cases. The virus also infected two pigs on a small Oregon farm, particularly alarming scientists. Pigs are often considered "mixing vessels" for flu viruses, helping new strains from birds adapt to mammals or acquire genes from other strains, creating "reassortant" viruses that pose a pandemic risk to humans.
The Pacific Flyway has been the site of numerous H5N1 outbreaks this fall. Source
The Pacific Flyway epidemic is also extending into neighboring Canada. Animal outbreaks have occurred in British Columbia and Alberta, resulting in Canada's first human case—a previously healthy teenager in critical condition. Similar to the Missouri case, no source of infection has been identified, and mutations that could increase the risk to humans were present.
Outside the US, other countries are dealing with their own H5N1 outbreaks. In Cambodia, a new strain of the virus is notably infecting more people, a stark reminder of the global scale of the H5N1 threat.
Meanwhile, in the US and Canada, winter is starting, and so is the cold and flu season. More frigid weather provides better conditions for flu viruses like H5N1 to spread, and outbreaks of other respiratory viruses like RSV and COVID-19 will make diagnosing cases harder. At the same time, co-circulation of H5N1 and seasonal flu increases the risk that the two viruses collide, creating a more dangerous hybrid strain. The CDC is working to vaccinate farm workers to mitigate the risk of such an event, but with early studies showing that this year's flu shots have reduced effectiveness, this is far from comforting.
The escalating situation has reignited long-standing concerns over pandemic preparedness in the United States. Despite numerous warnings and past experiences, the country was ill-prepared for COVID-19. With a clearly dangerous new virus spreading on American farms, the government is once again floundering. The country could easily be devastated if H5N1 mutates to be anything like the flu of 1918 or even COVID-19. In that case, the US and the world would have to turn to our one advantage over H5N1: years of advanced notice. But what have we done with this time?
For one, several candidate vaccines have been developed for H5N1. The US is moving to stockpile ten million H5N1 vaccines by Spring of 2025, though this would protect less than 2% of the population under a two-dose regimen. In the case of a pandemic, it would still take time to conduct human trials and ramp up production—and that's if the virus doesn't mutate and force researchers to start over. As seen in the Missouri patient, single mutations can make this happen.
The antiviral drug Tamiflu is a key part of US efforts to prepare for a future H5N1 pandemic. Source
Stockpiles of antiviral medications like oseltamivir (Tamiflu) could also help mitigate the spread of a hypothetical H5N1 pandemic. However, concerns exist over their effectiveness and how they would work against new mutations. For example, mutated viruses isolated from recent human cases in Washington may be more resistant to antiviral treatment.
Internationally, the potential for H5N1 to cause a global pandemic has led to growing unease. The World Health Organization (WHO) has urged countries to ramp up surveillance efforts and prepare for the worst-case scenario. However, challenges remain. While some nations have robust disease surveillance systems, others—particularly those in low- and middle-income regions—lack the resources to monitor new diseases effectively. Of course, even in the US, where such resources exist, authorities struggle. Whether H5N1 itself sparks a pandemic or not, a virus like it will. If the US (currently the most pandemic-ready country in the world) cannot stop H5N1, the implications for the next outbreak are chilling.
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