Bird flu cases are down, but the threat hasn’t passed. What to know about the H5N1 outbreak.

By most indications, the avian influenza outbreak that has washed over U.S. farms is at an ebb. Experts don’t seem hopeful, however, that the problem has taken care of itself.

The waterbirds that carry the virus in the wild have only just begun their spring migration, and peak migration season in April and May will bring renewed risks for new infections to hit chicken farms and dairy herds in the U.S.

Meanwhile, there have been new reports of cats sick with bird flu in New York City, cases in live poultry markets in Philadelphia, and warnings from a United Nations agency that the global spread of the virus is leading to “serious impacts on food security and food supply."

Here’s what you need to know about the status of the H5N1 avian flu outbreak in the U.S., and what it means for you.

Is the pace of new infections on farms slowing down?

It appears to be, for now. Farmers have only had to destroy two million birds due to H5N1 infections so far in March, down from 12.7 million in February and 23.2 million in January.

On dairy farms, the number of new infections is down, too. According to a Barron’s analysis of U.S. Department of Agriculture data, there have been a dozen dairy farms with new avian influenza infections confirmed in March, down from 17 in February, and 43 in December.

Those numbers are far below the explosion in cases in the fall and early winter, when the virus tore through California dairy herds. There were 228 new dairy farms infected across the U.S. in December, and 254 in November.

What’s behind this apparent lull in new cases?

One reason could be that there are fewer wild birds passing over farm country.

From November through March, birds are busy wintering in warmer regions to the south. Many of the poultry infections have happened when a migrating bird passes the virus to a commercial flock. Fewer migrating birds means fewer infected chickens.

The dairy farm cases likely have less to do with the volume of migratory birds passing overhead. Most infections on dairy farms likely trace back to a single instance in late 2023 or early 2024, when a bird passed the virus to a cow in Texas.

For dairy farms, it may be that after the virus’s explosive spread in California in the fall, there are simply fewer new farms to infect.

Though the number of new infections has dropped, plenty of dairy farms still have sick cows: According to the California agriculture department, of the 755 dairies infected in the state, 422 are considered completely recovered, which implies that 333 still have active infections.

What’s the risk of an H5N1 pandemic?

Experts disagree about how concerned they are about H5N1.

The Centers for Disease Control and Prevention says that the current public health risk is “low," though the risk to people exposed to infected animals is “moderate to high."

When Harvard Medical School recently asked six experts to rate how “worried" they are about H5N1 “becoming a major pandemic," their answers ranged between four and seven on a scale of one to 10.

The CDC has confirmed 70 cases of bird flu in the U.S. since the start of the outbreak, though that number is likely a significant undercount of the actual total: A recent study of blood samples of large animal veterinarians by CDC scientists found a handful with evidence of recent H5N1 infection who had not known they were sick.

One person is known to have died of bird flu in the U.S.

Why have there not been more deaths?

It’s a puzzle. In the roughly 20 years since H5N1 was first identified, about half of the people known to be infected with the virus have died. That frightening mortality rate is why scientists have tracked the virus so closely, and why it has been such a worry. But so far in the U.S., only one of the 70 people known to be infected has died.

The CDC put forward one theory last week that could explain what’s going on. The agency said that in studies in ferrets, animals that had previously been infected with the strain of seasonal flu known as A(H1N1) got less sick when they were infected with H5N1.

A(H1N1) is the flu virus that caused a 2009 pandemic, and is now a common seasonal flu. “While more study is needed, pre-existing antibodies could contribute to decreasing the severity of H5 bird flu illness in U.S. cases," the CDC said.

Are chicken products, eggs, milk, and beef safe?

Yes. The Food and Drug Administration says that pasteurization works against H5N1, that pasteurized dairy products are safe, and that eggs in the retail market are safe. The USDA says that the beef supply is safe, and that properly prepared and cooked poultry is safe.

The FDA says that while aging alone may not be enough to eliminate H5N1 in raw milk cheese, it knows of no one who got sick with H5N1 from eating aged raw milk cheese.

One concern may be raw pet food. Health authorities in New York City told New Yorkers not to feed their pets raw pet food from a particular raw pet food manufacturer, and that manufacturer has issued a recall of a raw cat food product.

What is the Trump administration doing about bird flu?

The USDA has announced new initiatives in recent weeks, including $500 million in new funding for biosecurity and $400 million in financial support for farmers. The USDA also said it would spend $100 million on other measures, including animal vaccine research. On March 20, the agency said it was taking applications for grants to develop new poultry vaccines for avian influenza, along with new therapeutics and diagnostics to treat the virus.

What else could the federal government be doing?

Vaccinating chickens. In an essay in the healthcare news website STAT on Thursday, Dr. Scott Gottlieb, a former FDA commissioner and senior fellow at the American Enterprise Institute who serves on the boards of Pfizer and Illumina, said a “sensible first step" would be to “seriously consider strategies for vaccinating egg-laying hens."

As Barron’s reported in 2023, there are logistical and technical challenges that would go along with a campaign to vaccinate hens. But vaccines exist, other countries do vaccinate chickens against H5N1, and biosecurity alone seems to not be enough.

Write to Josh Nathan-Kazis at josh.nathan-kazis@barrons.com