Will New Vaccine Recommendations Affect Your Fall Flu Shot?

Doctors and infectious disease experts stress that flu vaccines are safe, and they are especially important for older adults

Rachel Nania,

AARP

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En español

Published July 24, 2025

vaccination syringes placed around a red sphere

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Key takeaways

Recommendations for thimerosal-free may be confusing.
Thimerosal is a preservative used in multidose vials.
Almost 95 percent of flu vaccines haven’t used it.
No scientific evidence it has caused problems.
Older adults are more susceptible to flu, need the shots.

Federal vaccine advisers are recommending that nearly everyone roll up their sleeves for a flu shot this fall. This same recommendation has been made for more than a decade to help prevent illness from the common respiratory bug that killed as many as 130,000 people in this country’s most recent flu season.

But this year’s influenza guidance comes with a twist: The Advisory Committee on Immunization Practices (ACIP), with all new members that Health and Human Services (HHS) Secretary Robert F. Kennedy Jr. recently appointed, said June 26 that people should get a flu vaccine free from thimerosal, leaving many to wonder what that ingredient is and how it might affect their annual trip to the doctor’s office or pharmacy.

The good news: This recommendation, officially accepted as federal health policy on July 23, “is going to affect almost no one,” says Jodie Guest, an infectious disease expert and professor in the Department of Epidemiology at Emory University’s Rollins School of Public Health. That’s because the vast majority of flu shots given in the U.S. are already thimerosal-free.

You don’t need to ask for a thimerosal-free flu shot or worry they will be in short supply this fall, says Amesh Adalja, M.D., a senior scholar at the Johns Hopkins Center for Health Security and an adjunct assistant professor at the Johns Hopkins Bloomberg School of Public Health. “Literally, this is a nonissue,” he says.

Here’s what you need to know about thimerosal and why a flu shot is so crucial for older adults.

What is thimerosal?

Thimerosal is a mercury-based preservative used for decades to help prevent vials containing more than one dose of vaccine from becoming contaminated. Multidose vials are commonly used in developing countries, where resources are limited.

“It was put into our vaccines in order to keep them safe because at one point in time, there were cases of bacterial infections from multidose styles of vaccines,” Guest says. “This is something that was added in order to make sure that can’t happen, that you don’t get a bacterial infection from a vaccine.”

Thimerosal contains ethylmercury, which is different from the mercury found in certain fish, known as methylmercury, that can be toxic at high levels, according to the U.S. Centers for Disease Control and Prevention (CDC).

These different forms of mercury differ by one carbon atom, says Bruce Gellin, M.D., an infectious disease specialist, epidemiologist and senior adviser to Georgetown University’s Global Health Institute. “And molecules matter.”

Ethylmercury “is cleared from the human body more quickly than methylmercury and is therefore less likely to cause any harm,” the CDC says.

How common is thimerosal in our flu vaccines?

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About 94 percent of the U.S. vaccine supply for the 2024-25 flu season was thimerosal-free, according to the CDC. And thimerosal hasn’t been used in routine childhood vaccines since 2001.

“It’s only in multidose vials,” Guest says, “and so it’s actually going to affect very few of the [flu vaccine] vials that are out there.”

Multidose vials may be used where space is limited and you can’t store many single-use vials, Adalja says. “But it’s not something that you see being used very frequently at doctors’ offices or pharmacies.”

Vaccine manufacturers have confirmed that adult vaccine supplies will remain uninterrupted, according to an HHS news release.

Though the new recommendation won’t affect most people in the U.S., Cody Meissner, M.D., a member of the seven-person ACIP committee who opposed the vote to recommend only thimerosal-free flu shots, noted in the June 26 meeting that “the recommendations that the ACIP makes are followed [by] many countries around the world.”

“Removing thimerosal from all vaccines that are used in other countries, for example, is going to reduce access to these vaccines and increase cost,” Meissner added.

Globally, around 1 billion cases of seasonal influenza occur annually, including 3 to 5 million cases of severe illness, the World Health Organization estimates.

Is thimerosal safe?

Scientists have been studying thimerosal in vaccines for many years, the CDC says, and they have found no evidence it causes harm.

“Thimerosal use in vaccines and other medical products has a record of being very safe,” the public health agency says.

Research has not established a link between the preservative and autism or evidence of neuropsychological effects. Still, as a precaution, the U.S. started phasing out its use in childhood vaccines decades ago.

That decision “was made not because there was any evidence of harm from thimerosal,” Meissner said during the June 26 ACIP meeting. “It was made in an effort … to reduce the total exposure to mercury in our environment. That’s a reasonable objective.”

He continued, “The ACIP makes recommendations based on scientific evidence as much as possible, and there is no scientific evidence that thimerosal has caused a problem.”

Expert groups such as the American College of Physicians, the American College of Obstetricians and Gynecologists, and the National Foundation for Infectious Diseases (NFID) issued statements criticizing this break from the usual scientific process.

“There is no new scientific evidence warranting a change in recommendations for thimerosal-containing flu vaccines,” the NFID said in its statement.

Experts stress the importance of flu shots for older adults

Flu hospitalizations last year were the highest in more than a decade, new data shows. The virus sent an estimated 770,000 people to the hospital during the 2024-2025 season. Without vaccines, CDC scientists estimated, an additional 240,000 hospitalizations would have occurred, 170,000 in adults ages 65 and older.

“The virtually nonexistent risks of thimerosal in flu vaccines pale in comparison to the real risks posed by flu each year in the U.S.,” the NFID said in its statement.

Adalja says, “There’s been zero documented risk from thimerosal, whereas we know that flu will kill tens of thousands of people every year, hospitalize hundreds of thousands and infect millions in the United States. So there’s no comparison in the risks. One is zero, and one is significant.”

Experts recommend that older adults, who are more susceptible to the flu and its complications, receive a high-dose version of the flu shot for extra protection.

Between 70 and 85 percent of seasonal flu–related deaths occur in adults 65 and older, the CDC says. “Flu shots are safe, and they are a great way to stay safe when the flu season begins,” Emory University’s Guest says.

The CDC says the best time for most people to get vaccinated is September and October.

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