Bird Flu: Letting the Virus Fly Wild Is Probably Not a Good Idea

A Scientific Examination of the "Let It Rip" Approach to H5N1 Management

When Health and Human Services Secretary RFK Jr. suggested a dramatically different approach to managing highly pathogenic avian influenza (HPAI) H5N1 on American farms, the scientific community took notice. Instead of culling infected poultry flocks—the standard practice for decades— Kennedy proposed letting the virus "run through the flock to identify the birds, and preserve the birds, that are immune to it." Agriculture Secretary Brooke Rollins has echoed support, mentioning that some farmers are willing to pilot this strategy.

There could be two things RFK Jr. is alluding to here: 1) some sort of genetic trait that makes chickens resistant to infection in the first place or 2) those birds "strong enough" to survive an infection. Both of these are problematic due to the spreading of infection to other birds and for other reasons we detail below.

It's a hypothesis that merits scientific examination: Could natural selection identify H5N1-resistant poultry? Let's analyze the biological realities that make this approach problematic.

Let’s discuss…

Situation Report on Bird Flu in the US

The H5N1 avian influenza outbreak continues to spread in both animal and human populations. According to CDC data from March 19, 2025, there have been 70 human cases since April 2024, with 41 associated with exposure to infected dairy cows, 26 linked to poultry exposure, and 3 cases of undetermined origin. While human-to-human transmission has not been documented and risk to the general public remains low, the animal impact is substantial: 989 dairy herds across 17 states have confirmed H5N1 infections, while 336 commercial poultry flocks and 207 backyard flocks have been affected, impacting over 90.9 million birds.

The spread appears to be continuing rather than "burning out." Epidemiological data shows at least three separate documented cases of wild bird-to-dairy cow transmission (in Texas, Nevada, and Arizona), indicating multiple entry points for the virus. Further complicating control efforts, preliminary evidence suggests that cows can become reinfected, creating potential ongoing transmission cycles in dairy herds.

Surveillance testing indicates we're likely missing milder human cases. When CDC researchers tested blood samples from 150 bovine veterinarians who had not previously tested positive for H5N1, three had antibodies indicating prior infection they were unaware of having experienced.

Dr. Liz Marnik recently did a reel on this topic which is a fantastic primer– and now let’s get into the weeds!

sciencewhizliz

A post shared by @sciencewhizliz

Dr. Jenn Dowd also has a recent Substack on this topic as well, you can read it here.

Genetic Homogeneity in Commercial Poultry Populations

The first biological constraint involves the limited genetic diversity in commercial poultry populations. For natural selection to be successful, the genetic variant must already exist in the population. Modern agricultural breeding practices have prioritized production traits over disease resistance, creating flocks with minimal genetic variation. "The way we raise birds now, there's not a lot of genetic variability. They're all the same bird, basically," notes Dr. Gail Hansen, a former state veterinarian.

This genetic uniformity is particularly relevant when considering disease resistance. Researchers studying other avian pathogens have found limited evidence for strong natural resistance factors. Professor Huaijun Zhou at UC Davis investigated Newcastle disease—another lethal avian virus that progresses more slowly than H5N1. His team identified no single genetic resistance marker but rather multiple genetic variations, each with minimal protective effects. Even with hundreds of such variations working in concert, they produced only modest differences in survival rates.

The immunogenetic research suggests that breeding for H5N1 resistance would require identifying and selecting for multiple genetic factors simultaneously—a complex undertaking even under controlled conditions, let alone through natural exposure to a rapidly fatal pathogen.

Biological Barriers to Pathogen-Generated Immunity Development

The primary obstacle to Kennedy's natural immunity hypothesis lies in the virus's extreme virulence and rapid progression. H5N1 kills 90-100% of infected chickens within just 72-96 hours through a systemic assault on multiple organ systems. This timeline is fundamentally incompatible with immune response development.

"It's devastating," explains poultry veterinarian Rocio Crespo. "The disease attacks every single organ."

For natural selection to work, organisms must survive long enough to reproduce. Avian H5N1 infection doesn't allow this opportunity. Most chickens infected with H5N1 die before their adaptive immune systems can mount an effective antibody response, which typically takes 7-10 days to develop. While it is theoretically possible some birds might possess genetic factors providing partial resistance to infection or disease progression, the rapid progression of infection we see among flocks makes this unlikely. Most birds cannot survive, suggesting there is not a common genetic factor to easily select for. Additionally, birds that might theoretically survive infection would represent such a small percentage of the population that building a resistant flock through this method would be impractical at commercial scale.

Chicken Economics 101: Why "Let It Rip" Would Ripple Through Your Grocery Bill

Understanding the structure of the poultry industry reveals another flaw in the let-the-virus-run-wild approach. The chickens on commercial farms—the ones that would be exposed to H5N1—aren't breeders. They're the end product, like the car that rolls off the assembly line rather than the factory that builds it.

As virologist Matt Koci points out, wiping out these production flocks wouldn't do anything to alter the genetics of future generations. The breeding happens elsewhere, in specialized facilities with birds that are selected for specific traits and carefully protected from disease.

The economic ripple effects would be immediate and severe. Other countries would almost certainly ban U.S. poultry imports if we adopted a "let it rip" strategy. Domestic egg and chicken prices would skyrocket as supplies dwindled.

"The Chick-fil-A's and the Kentucky Fried Chickens and all the chicken dinners you have, forget it," warns Crespo, "gone." (Maybe thinking about all the crispy chicken sandos we would no longer be able to eat would help get the point across?)

In an era of already uncomfortable food inflation, the impact on consumers would be painful and immediate.

Note for folks with backyard chickens– please make sure you wear appropriate PPE and take appropriate precautions. (Check out the CDC page for more.)

Virological Risk Assessment: Mutation and Reassortment Potential

The most significant scientific concern with allowing widespread H5N1 infection involves viral evolution and pandemic potential. Influenza viruses mutate through two primary mechanisms: antigenic drift (gradual accumulation of point mutations) and antigenic shift (genetic reassortment between different influenza strains, Liz has an illustration of this here).

H5N1 has already demonstrated the ability to cross species barriers, infecting dairy cows, various wild mammals, and humans. While the current strain causes predominantly mild symptoms in humans, virologists warn that increased transmission provides more opportunities for adaptation.

"Any strain of H5N1 has the potential to reassort with another strain of influenza and become something more dangerous," explains Dr. Amesh Adalja of Johns Hopkins. During reassortment, viral gene segments from different influenza viruses can combine when two influenza viruses infect the same cell, potentially creating novel variants with enhanced transmissibility or virulence.

Dr. James Lawler from the University of Nebraska quantifies this risk: "Every new infection in an animal and person is like throwing a pair of thousand-sided dice. It may take a while, but eventually, it will come up snake-eyes." If a single commercial flock of five million birds were infected, that would create "literally five million chances for that virus to replicate or to mutate," explains Dr. Hansen.

Historical context is important: the 1918 influenza pandemic had a case fatality rate of approximately 2% yet caused an estimated 50 million deaths globally. As Dr. Lawler notes, it was "one of the worst pandemics in recorded history." Virulence and transmissibility operate independently, meaning a virus need not be highly lethal to cause significant public health impact if it spreads efficiently.

So What's The Better Path Forward?

If letting the virus spread naturally isn't the answer, what is? Scientists are focusing on several more promising approaches:

1. Enhanced biosecurity to prevent the virus from entering farms in the first place. This includes strict sanitation protocols, controlled access to poultry houses, and monitoring systems to detect outbreaks quickly.

2. Vaccine research that focuses on how different genetic lines of poultry respond to vaccination. Currently, vaccines can keep chickens alive but don't prevent infection entirely, which creates trade complications. Countries don’t want to accept birds that may be infected without them realizing it. Understanding why some birds' immune systems respond better than others could lead to more effective vaccines.

3. Continued targeted culling when outbreaks occur. While painful, this approach has successfully controlled bird flu outbreaks in the U.S. since the 1980s.

Emerging Research on H5N1

Recent studies are providing important insights into H5N1 biology and potential countermeasures:

1. Cross-reactive immunity: CDC research published in Emerging Infectious Diseases (February 2025) found that ferrets previously infected with seasonal influenza A(H1N1)pdm09 developed cross-reactive antibodies that provided partial protection against H5N1 infection. Individuals vaccinated for influenza over several years in the past and/or those who have been infected with influenza may have some level of cross-reactive immune protection. This may explain why most U.S. human cases have been mild despite the virus's historically high mortality rate globally.

2. Transmission routes: A study published March 17, 2025, in The Lancet Microbe demonstrated that ferrets could develop severe, transmissible disease from H5N1 exposure via eye surfaces, highlighting the importance of comprehensive personal protective equipment for those working with infected animals. (Does this conjure the image of the Monsters Inc. clean-up crew for anyone else? Just us?)

3. Antiviral susceptibility: CDC research (March 7, 2025) confirmed that current H5N1 strains remain susceptible to neuraminidase inhibitors like oseltamivir (aka Tamiflu), though with somewhat reduced sensitivity compared to seasonal influenza viruses. The laboratory findings do not warrant changing current treatment recommendations.

These findings underscore the importance of continued research while maintaining proven containment strategies.

Scientific Consensus: Biological Realities Preclude Natural Immunity Approach

The scientific examination of the "let it rip" hypothesis reveals multiple biological constraints that make it untenable as a management strategy for H5N1. The extreme virulence and rapid progression of H5N1 infection in poultry creates a fundamental biological contradiction: birds cannot survive long enough to develop natural immunity. The limited genetic diversity in commercial poultry populations further constrains the potential for identifying resistant lineages. In a nutshell (...or an eggshell?), because bird flu is so lethal to poultry, there is no reproduction (aka dead birds can’t make babies).

The proposal also presents substantial risks: agricultural economic disruption, animal welfare concerns, and increased opportunities for viral evolution toward variants with enhanced human transmissibility. Evidence-based approaches to H5N1 management must acknowledge these biological realities.

As we navigate the complex challenges of highly pathogenic avian influenza, solutions that incorporate enhanced biosecurity, targeted culling, and continued research into vaccines and antivirals represent our most scientifically sound strategy against a pathogen that continues to present significant agricultural and public health challenges.

Stay Curious,

Unbiased Science and Science Whiz Liz

P.S. Want to support this kind of analysis? The best way is to subscribe to our Substack and share our work. While all our articles are always completely free to read, paid subscriptions help sustain our in-depth reporting on vaccine policy and public health. Thank you for considering it!