A “small but detectable quantity” of infectious H5N1 bird flu virus was able to survive a common approach to pasteurizing milk, according to new research co-authored by scientists at the National Institutes of Health. 

The findings, published Friday in The New England Journal of Medicine, were based on experiments run at the agency’s lab. The researchers note this is not the same as finding infectious H5N1 virus in milk from grocery stores. 

So far, officials have not detected infectious virus in any supermarket milk samples.

The finding comes as authorities are still identifying new infected herds in this year’s unprecedented outbreak of H5N1 bird flu in dairy cattle. 

Infections have been detected in cows across farms in at least a dozen states, with most of the positive tests coming from raw milk samples that were teeming with the virus. Authorities have called on states to curb raw milk sales that could spread the virus, and have warned consumers against drinking raw milk.

“The study reflected experimental conditions, and should not be used to draw any conclusions about the safety of the U.S. milk supply,” a Food and Drug Administration spokesperson said in a statement.

In the real world of commercial dairy processing, milk from infectious cows is likely being mixed with milk from healthy cows, diluting the virus and making it less likely there would be enough of it to survive. Technical details of how the milk is pasteurized, as well as additional steps to treat the milk, also cut the risk. Pasteurization involves treating the milk at high temperatures for a period of time to kill contamination.

Results from previous FDA studies of 297 samples of retail dairy products like milk and yogurt did not turn up any infectious virus. Earlier tests found only some harmless fragments of the virus leftover from pasteurization.

Experimental laboratory conditions

“These are more or less experimental laboratory conditions. And we think that mechanical pasteurization in dairy farming will probably be more effective than what we do,” said Vincent Munster, chief of the virus ecology unit at the National Institute of Allergy and Infectious Diseases.

Munster’s lab in Montana has done work for years studying the ability of viruses to stay infectious in various conditions, including previous work pasteurizing pathogens in milk, like from an outbreak of coronaviruses in camels from Jordan. 

In this study, they looked at two approaches of pasteurization, tested on milk with concentrations of H5N1 they added at levels similar to what has been seen from some highly infectious cows. 

One pasteurization method they looked at heated milk to 72 degrees Celsius (about 161 degrees Fahrenheit) for 15 seconds, similar to the “high temperature short time” method that is used widely across the dairy industry. That yielded milk with what Munster said still had “minute amounts of infectious virus.”

“You’re really talking about like 10 virus particles, whereas the initial starting dose would be something like 10 million or 100 million virus particles,” said Munster.

He also pointed to differences between the lab study and real-world industry practices which give commercial milk pasteurization a step up against the virus.

“There is an initial period in the dairy milk pasteurization that the milk needs to get from let’s say 4 degrees to 72 degrees [Celsius]. And obviously once it starts hitting around 56 degrees, it already starts inactivating the virus,” he said.

The second pasteurization approach — a half hour at 63 degrees Celsius — was more effective. Infectious virus was undetectable within 20 seconds, long before pasteurization was over.

“Pasteurization methods were developed to actually reduce the amount of viable bacteria in milk to prolong fridge life, so to speak. And they’re not necessarily initially designed to inactivate viruses,” said Munster.

Pasteurizing milk longer or at hotter temperatures

Munster thinks that even adding 5 to 10 more seconds of pasteurization could offer the dairy industry a “safety buffer,” ensuring that there’s no active virus leftover in milk even if their raw milk supply turns out to have greater concentrations of infectious virus than the lab’s .

“If you really want to make 100% sure there’s no active virus, increasing the duration even by like 5 to 10 seconds of pasteurization would allow you to actually increase that safety margin,” Munster said.

But an FDA spokesperson said that their testing data so far shows the pasteurization processes used by U.S. dairy companies are effective at killing H5N1. Many companies “use temperatures that are greater, often much greater than the minimum standards,” the spokesperson said, and equipment that more consistently heats milk.

“[T]he United States would hesitate to change pasteurization parameters without data to demonstrate a public health need,” the spokesperson said, warning that changing the standards would affect the flavor of dairy products.

The agency has so far not released results from its own study to validate the pasteurization of raw milk for H5N1, first announced earlier this year. Last month, it cited “the totality of the evidence” in reiterating that “the commercial milk supply is safe.”

The spokesperson said the FDA’s study “is a top priority for the agency” and that they were working to share the results in the near future. The agency is looking to validate “real-world processing conditions” with equipment used in commercial facilities.

“Sound science is critical to informing public health decisions like those made by the FDA related to food safety and we take this current situation and the safety of the milk supply very seriously,” the spokesperson said.

Munster said the FDA and the U.S. Department of Agriculture also have studies looking at pasteurization, among a number of groups that have confirmed heating up milk is often able to fully inactivate the threat from H5N1 virus.

“Fortunately, they do actually inactivate viruses very well. But I think the focus point should be two fold: making sure that pasteurization is up to the task we’re asking from it, and the data suggests that it is, but also that we should minimize any H5N1 positive milk actually coming into these dairy pasteurization,” he said.