For the past week, about 50 flu scientists from around the world have been cramming into a conference room at a Hilton hotel in Istanbul, Turkey.
Their goal is to design a flu shot that will confer the best protection for the next flu season —starting in the fall of 2026. Each day, they pore over reams of data — about how the virus is evolving worldwide, how well last year's shot performed, and which strains might be easiest to mass produce for a vaccine.
The meeting, convened by the World Health Organization twice a year, is a critical moment for the WHO's Global Influenza Surveillance and Response System.
It's also "really tedious," says Dr. Dan Jernigan, who led the Centers for Disease Control and Prevention National Center for Emerging and Zoonotic Infectious Diseases from 2023 to 2025. "In order for you to make the best choice for what to put in the vaccine, you have to review a lot."
In past years, CDC scientists like Jernigan have played a major role in these meetings. But after the U.S. officially withdrew from the WHO in January, it wasn't clear the U.S. would participate in any WHO-led meeting. Earlier this month, the administration confirmed CDC would be sending staff despite the U.S. exit from the organization, albeit virtually instead of in person.
"CDC representatives will take part in the meeting to support international technical collaboration," the Department of Health and Human Services said in a statement to NPR. "Their participation will solely focus on providing technical expertise, sharing surveillance data, and contributing to scientific discussions that inform vaccine strain recommendations. This participation does not change the U.S. position on withdrawing from the WHO."
"I think it speaks to the irreplaceable nature of these multi-country networks," says epidemiologist Jennifer Nuzzo, director of the Pandemic Center at Brown University. The Trump administration "may bluster and say, 'We're not going to do this,' but at some point, they run into reality, which is that there is no other way to protect the nation."
Still, the U.S. withdrawal from the WHO has challenged this global system and could ultimately diminish the influence of the U.S. in shaping the flu vaccine.
Collecting global data
At any given moment, flu is spreading somewhere on Earth. Each year, roughly 1 billion people are infected. And year-round, the WHO's surveillance system collects data from those sick patients in 130 countries.
Those samples are then sent to seven larger labs — including the CDC in the United States. They're referred to as WHO collaborating centers, and they try to make sense of all that virus data and select strains that might be suitable for vaccines.
That global system depends on the constant flow of samples from many countries to those seven bigger labs. WHO foots the bill for the shipment of samples, but when its biggest funder — the U.S. — pulled out last year, the flow of influenza samples slowed down in large part because of the loss of U.S. dollars. Fewer samples means a dimmer view of influenza's evolution, which could make it harder to know which strains to put in next season's vaccine.
"There was a slight dip in the circulation of influenza vaccines around the world because of a challenge with some funding," said Maria Van Kerkhove, interim director of the department of epidemic and pandemic threat management at WHO, at a February 11 press conference. "But we have been able to resume the shipments around the world."
Still, Jernigan worries about the long-term viability of the system, given WHO's funding constraints. The recent uptick "does not mean things are back to normal," he says.
Another departure from normality is the absence of U.S. officials literally at the conference room table.
"You want the whole process to be very objective and quantitative, but ultimately the interaction of the different researchers is really important," says Jernigan. CDC researchers have always had a lot of sway in picking which strains to include in the vaccine. But with U.S. officials only participating virtually, and the U.S. shunning international collaboration more generally, representatives from other countries may be less inclined to follow America's lead.
"You want your country's problems to be represented in what flu virus gets selected" to ensure the best match, says Jernigan. With the U.S. in virtual attendance only, he says, "there's really not an incentive for them to select the vaccine virus that represents what may be circulating in the United States."
NPR asked the Department of Health and Human Services, which houses CDC, to address that concern but the agency did not provide a response.
Friday morning, WHO will announce the strains recommended by the committee for next year's flu shot. Then manufacturers start the production process, which takes about nine months until the shots are ready to go.
"It's the ultimate multilateral product of a whole global system," says Dr. Ali Khan, dean of public health at the University of Nebraska. "It's reassuring at least that the CDC will be at these meetings," he says, but "participating or not, we're seeing a bleeding of influence of the U.S. government in global health."
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