As bird flu cases in the United States have begun to surface over the past year, public health officials continue to raise concern over the potential rapid spread of the virus among humans.
The circulating H5N1 strain of the bird flu has resulted in 67 total confirmed cases and one death in the United States since the outbreak in 2024, according to the Centers for Disease Control.
While the presence of the H5N1 virus isn’t new — there have been 970 confirmed human cases across the globe since 1997, the CDC reports — there has yet to be human-to-human transmission of the virus. But mutations in the H5N1 strain over the past two years have concerned public health experts as the virus could potentially be more contagious between humans.
“What has changed in the last two years is that first, it has become way more widespread among birds, but second, we are seeing large-scale infection of mammals,” said Ashish Jha, the dean of the Brown University School of Public Health and former White House COVID-19 response coordinator.
Historically, H5N1 primarily caused outbreaks in wild birds and occasionally in poultry. But as of March 2024, the bird flu has spread to over 900 herds of dairy cows across 16 states.
The more H5N1 is transmitted around humans, the higher the risk of human-to-human transmission, which can end up being “very, very deadly,” Jha said.
Experts have studied this virus for more than 25 years, but it remains difficult to predict if viral evolution will occur, let alone when the virus might mutate, said Jennifer Nuzzo, an epidemiologist and the director of the Pandemic Center at the Brown University School of Public Health.
“Just because it hasn't happened doesn’t mean it won’t,” Nuzzo said, adding that mutations in the disease indicate that infection in humans is “increasingly possible.”
A majority of the contracted cases in the United States come from farm workers exposed to cattle. While most of these cases result in mild symptoms such as eye redness and fever, this may be because the virus has primarily infected young, healthy farm workers and not that the virus itself has become less lethal, according to a Science News article. Historically, H5N1 has had a mortality rate of approximately 50%.
Jha pointed out that most bird flu cases in the US have been contracted through direct contact with infected cows, rather than through airborne transmission from birds.
“I don’t think this virus has become any milder. It’s just the way that it has transmitted has not caused the same level of serious illness so far,” Jha said. “But that is not the same thing as saying it will not become a more serious problem over time.”
CDC guidelines recommend avoiding exposure to sick or dead animals and wearing personal protective equipment if contact with these animals cannot be avoided. As some raw milk samples have been found to be contaminated with H5N1, the CDC also urges the public to only drink pasteurized milk and to cook products containing beef, eggs and poultry to a safe temperature.
Amanda Jamieson, an associate professor of molecular microbiology and immunology, told The Herald that having updated vaccines in stockpile and tracking the spread of the virus will be critical to mitigating any potential spread of H5N1.
The federal government currently has up to 10 million doses of a bird flu vaccine for emergency use, which are yet to be made available to the general public. Moderna was recently awarded $590 million from the U.S. Department of Health and Human Services to accelerate the development of mRNA-based vaccines to target current strains.
Challenges in tracking the virus largely arise from a lack of access to testing, which Nuzzo says is necessary to determine whether an infected individual suffers from H5N1 or the seasonal flu. Nuzzo expressed support for increased testing availability at farms so workers can “make their own judgments” on isolating animals, protecting workers and seeking medical help.
“The overarching lesson is that we’re going to see more and more of these pathogens emerge and potentially go on to cause pandemic spread,” Nuzzo said, “and so we have to just be ready for them like the way we get ready for other recurring hazards like natural disasters.”
Unlike COVID-19, which emerged suddenly and provided authorities with less time to ramp up protections, Jha said a lack of preparation is not an excuse this time around, given that H5N1 has been in circulation and studied for decades.
“I don’t think (the bird flu) is now at a point where most individuals need to be taking any steps,” Jha said, adding the University will continue monitoring the progression of the virus but will not be taking any specific steps at this time. “This is really a time where public health and government need to be taking action."
Jonathan Kim is a senior staff writer covering Science and Research. He is a first-year student from Culver City, California planning to study Public Health or Health and Human Biology. In his free time, you can find him going for a run, working on the NYT crossword or following the Dodgers.
