Update 1/29/2025: A new strain of bird flu—H5N9—was found at a duck farm in California. It is the first confirmed case of this strain in the United States.
What is going on with bird flu?
I cannot be alone in finding some of the rhetoric around it confusing. On one hand: almost daily there seem to be concerning, almost panic-inducing, headlines. Things like “How America lost control of the bird flu and raised the risk of another pandemic” and “First severe human case of bird flu in U.S. shows ‘concerning’ mutation.”
On the other hand, the imminent-danger headlines have been there for months or more. Back in June, the New York Times published an article entitled “Why the New Human Case of Bird Flu Is So Alarming.” Whatever imminent dangers that was warning of, it doesn’t seem to have come to pass. And the CDC still indicates that the risk of a bird flu pandemic is low.

Back on the first hand, California has declared a state of emergency and warned people strongly against raw milk. But on the second hand, it isn’t clear any people have gotten bird flu from milk, even though bird flu has been detected in some raw milk.
The reason for the back-and-forth here is, broadly, that there is a concern but not imminent danger. To understand why, it’s useful to go back to basics, which is where we are today. My goal here is to make the inevitable future headlines on this a little more decipherable.
What is “bird flu”?
Influenza viruses are labeled based on two protein types on the surface of the virus — the type of hemagglutinin (H) protein and the type of neuraminidase (N) protein. H5N1 has a type 5 in the H position and a type 1 in the N position. The particular type of protein affects the behavior of the virus — who it can infect, how dangerous it is, etc.
Human seasonal flus are generally H1N1 or H3N2. (These are all variants of influenza virus A, which is distinct from a separate, more mild, influenza B virus.) There are a lot of additional variations within these broad subtypes, which is why human seasonal flu vaccines are updated each year.
The H5N1 flu type is commonly found in birds (hence the term “avian flu” or “bird flu”), but it can infect humans and other animals.
When we think about our level of concern about viruses in general, we worry about two things. First: how serious is the virus? Second: how easy is it to get?
How serious is H5N1?
The reason H5N1 causes concern is that this virus appears to cause much more serious illness in humans. It’s not entirely clear why — this article has a good overview and suggests it has to do with how it binds in the respiratory system. Current estimates indicate a mortality rate of 50% from H5N1 (which would be vastly, vastly higher than seasonal flu), although that is likely an overestimate for what we would see if infection was widespread. Still, it seems clear the virus is more dangerous, which is why there is so much concern.
How infectious is H5N1?
H5N1 has not, thus far, been very infectious to humans. Humans can get the virus, but human-to-human transmission does not seem to have occurred. And moreover, most of the human infections have been in individuals with a lot of contact with infected birds or cows. Casual bird contact does not seem to have been implicated.
This lack of infectiousness seems to reflect, at least in part, the virus being targeted for avian-type receptors, not human-type. The virus is just not well suited to infect people in its current form.
Implications and changes
In the current situation, H5N1 poses little threat to human populations because of the limited infectivity. A virus that cannot spread human-to-human cannot get a good foothold.
The primary concern with H5N1 is it could mutate to become more infectious. This paper points out a relatively small change that could allow the virus to match to human receptors. Influenza viruses mutate frequently, so this is a real concern. If a mutation makes the virus more infectious without making it less severe, this would completely change the situation.
Why are you hearing so much about it now?
Everything I have said up to this point has been true for decades. H5N1 has been known to pose a threat of serious illness in humans for a long time, and sporadic human cases have arisen many times. The CDC has been tracking cases since at least 1997.
What has changed in the past 18 months is that the virus has moved into dairy cattle. This is the first time that this is thought to have happened, and it suggests some mutation in the virus that makes it easier to infect cows. The degree of spread strongly supports cow-to-cow transmission, and in California in particular infection is widespread.
The mortality in cows is low thus far — estimated at perhaps 2%, which includes herd culling. There are two reasons why the move to cows raises concerns for people. One is that people have a lot more contact with cows than they do with wild birds. More contact means more risk, and indeed many of the individuals who have gotten H5N1 in the past 18 months were farmworkers. There is a worry that exposure is possible through raw milk, where the virus has been detected, although no illnesses have been linked to that yet.
The second concern is that cows are more similar to people than birds are, which means that the viral mutation that allowed cows to be infected could make human infection more likely.
So far, however, this hasn’t come to pass. The CDC tracks the H5N1 situation, and there have been 66 confirmed human cases and one death thus far. The individual who died was in Louisiana, and his case was detected in December 2024, likely from exposure to a backyard poultry flock. Overall, the current situation does not pose an immediate threat.
How would we know if things changed?
This is perhaps the key (and most difficult) question. As noted above, the thing that would change the situation is if the virus mutated to produce sustained human-to-human transmission. The unfortunate reality is that we might not find out about that until significant spread has happened, at which point it would be difficult to stop.
The CDC is very focused in its case investigations on whether human-to-human transmission has occurred. The hope is that the agency could identify changes in transmission patterns early. This is what we would look for to see if things change.
What would we do if there was a pandemic?
Human behavior and policy responses are hard to predict, so I will not venture a guess there. Have we learned from the successes and failures during COVID? Probably not. Policy responses would also vary tremendously depending on the level of infectiousness and how serious the disease is.
From a disease standpoint, the answer here is clearer, and somewhat more reassuring. In contrast to COVID-19, we are better prepared (not well prepared, but better prepared) for an H5N1 pandemic. There are a number of antiviral medications that we know work well against this virus. There are three H5N1 vaccines that have been approved in the U.S., and an mRNA vaccine is under development. More doses would need to be produced and some of these vaccines might need to be updated (and of course there are concerns about people being resistant to vaccination), but we would not be starting from zero, as we were with COVID.
The bottom line
- “Bird flu” is an H5N1 flu type that is commonly found in birds, but it can infect humans and other animals. The CDC has been tracking cases for decades. H5N1 has not, thus far, been very infectious to humans.
- Recently, bird flu has gotten more attention because it was detected in cows, suggesting a possible mutation.
- At this point, we are better prepared for a potential pandemic than we were with COVID-19. There are antiviral medications that we know work well against the virus and three H5N1 vaccines that have been approved in the U.S. There is not a reason to believe we would be starting from zero, as we were with COVID.
Log in
Is there evidence on how effective seasonal flu vaccination (or immunity through infection) is at protecting against H5N1?
Most of the panic headlines I’m seeing are 100% Mortality in Cats!!! due to raw meats (large cats in zoo settings and pets) and raw milk. Further panic articles regarding household pets include: human to feline transmission, their proximity to wild birds, droppings, and tracking in the virus on our shoes! While I know we are human health orientated here — can any light be shed on the worries for the furry members of our families?
I think raw milk has been linked to cats getting the disease. And it’s a lot more fatal in cats (has caused major problems for at least 1 large cat sanctuary). So it definitely seems feasible. We’re keeping a close eye on this one in the veterinary field
I’m wondering what you make of the fact that the 60odd cases so far have been mild, especially in light of you citing 50% mortality. Also, how much genotyping is happening? I don’t imagine CDC has the resources to be keeping up with that, so how is philanthropy stepping up? In my clinic, it’s up to us to flag a sample for genotyping based on animal exposures, which of course will not contribute to helping to detect person person.