The US Government has threatened to withdraw all future funding unless Gavi, the vaccine alliance, removes the ingredient thimerosal from its vaccines. This comes just as Gavi and its partners resume distribution of cholera vaccination supplies to several African countries.
NPR reports that central to the ultimatum from the US Department of Health and Human Services to the group, which helps provide vaccines to the world’s poorest countries, is an unsubstantiated theory that the mercury in thimerosal could be linked to autism.
Thimerosal is a chemical compound that is added to vaccines as a preservative.
This is the latest development in the changing relationship between Gavi, which has provided vaccines for 1.1bn children in lower-resource countries and prevented 18m deaths, and the US, once a major funder of the vaccine group.
In June, the Trump administration cut off more than $1bn previously promised to the group. At that time, HHS Secretary Robert F Kennedy Jr said that Gavi “ignores the science” when it comes to vaccine safety.
Now, thimerosal has become a new sticking point in the US-Gavi relationship.
Although thimerosal has been used safely in medications and vaccines since the 1930s, it’s also been the topic of a long-simmering debate that has reignited domestically in the past year.
With this latest demand from HHS, the controversy is going global. And, experts say, the potential impact is substantially greater abroad than in the US.
That’s because this ingredient is critical for preventing contamination of vaccines in vials containing multiple doses that are often used in the Global South.
“The stakes are much higher in low-income countries,” said Dr William Moss, a Professor at Johns Hopkins Bloomberg School of Public Health and executive director of the International Vaccine Access Centre.
Why and where is thimerosal used?
Thimerosal has a special antimicrobial property: it’s very good at preventing the growth of bacteria and fungi, so for decades, it’s been added to vaccines to help avoid contamination.
This is particularly important for multiple-dose vials. For example, one bottle might contain 10 doses of the given vaccine. When a syringe goes into a vial to draw up the liquid, there is a risk of introducing germs. The presence of thimerosal deters growth of any germs.
In the US, the use of thimerosal has “significantly declined”, according to the FDA, partly because vaccines are now packaged in single-dose vials.
Europe also relies heavily on single-dose vials. However, in many places supported by Gavi, multi-dose vials are regularly used.
“High-income countries can afford single-dose vials, which are more expensive, but more expensive to transport and store,” said Moss.
“But low-income countries have to maintain a cold chain and they just don’t have the refrigerator space to be able to store the single-dose vials.”
It’s common for low-income countries to use multi-dose vials for immunisation campaigns that last days or weeks, when a large number of children are vaccinated. “It’s very efficient and cost effective to use these multi-dose vials as long as they don’t get contaminated with bacteria or fungi – and that’s exactly what the thimerosal is doing,” Moss added.
Indeed, about 14% of Gavi’s portfolio consists of vaccines that contain thimerosal in multi-dose vials, including the 5-in-1 pentavalent vaccine, the DTP vaccine that protects against diphtheria, tetanus and pertussis and the hepatitis B vaccine.
What’s the fuss about?
Thimerosal is about 50% mercury by weight. That raises suspicions in some circles.
“People think of the fictional T-1000 from the movie Terminator 2 – liquid metal, that’s going to cause serious brain injuries,” said Angela Rasmussen, a virologist at the University of Saskatchewan and co-editor-in-chief of the scientific journal Vaccine.
In reality, she said, the type of mercury it contains is ethylmercury which quickly leaves the body and is not harmful in the amounts contained in vaccines.
Methylmercury is a more dangerous type of mercury and can accumulate in the body when people are exposed in their environment or in certain foods, like fish.
“The dangerous mercury that could be toxic from a single thimerosal-containing vaccine is less than you would get from eating a tuna fish sandwich,” she added.
Leading health institutions agree with Rasmussen: thimerosal has the blessing of the World Health Organisation, whose advisory committee found no evidence of toxicity in infants, children or adults exposed to thiomersal in vaccines.
Similarly, US federal agencies have deemed it safe. The Centres for Disease Control and Prevention says on its website that “research does not show any link between thimerosal in vaccines and autism”.
And the FDA has found that “the scientific evidence collected over the past 20+ years does not show any evidence of harm”.
However, Kennedy disagrees, and has long campaigned against thimerosal.
In a recent video, he said that the mercury found in thimerosal is a “very potent neurotoxin”, explaining his decision to order thimerosal removed from all influenza vaccines in America.
However, the decision didn’t have a huge impact because only about 6% of flu vaccines contained thimerosal.
Back in 2001, the US removed thimerosal from nearly all vaccines routinely given to children under six as a precautionary measure. The American Academy of Paediatrics said this decision was made because all the misleading claims about thimerosal were “eroding public trust in vaccine safety”, and it didn’t want to risk children going without their shots.
The CDC website says: “Even after thimerosal was removed from almost all childhood vaccines, autism rates continued to increase, which is the opposite of what would be expected if thimerosal caused autism.”
Status of the ultimatum
The US has asked Gavi for “a detailed action plan with a timeline for the phase-out of thimerosal-containing vaccines”.
“The goal is to bring the poorest countries in line with the standards in the US, Canada, and most European nations, who have long-since phased-out thimerosal-containing vaccines for children,” it says.
Gavi told NPR: “We remain in contact on this subject,” and any change “would require a decision by Gavi’s Board and will be guided by scientific consensus”.
Potential implications
Gavi has not indicated it would comply with the request. The HHS statement said, “Gavi has refused to develop this [phase-out] plan.”
Nonetheless, vaccine experts worry what could happen if Gavi did agree to eliminate thimerosal from its vaccines.
“It would make vaccines more expensive in the developing world, which means they will be available to fewer children,” said Dr Paul Offit, a paediatrician and director of the Vaccine Education Centre at the Children’s Hospital of Philadelphia.
He said the ultimate outcome is clear: “Children will no doubt die because those vaccines are unavailable.”
Jabs for Africa
Meanwhile, last week Gavi, the WHO and Unicef announced that the global cholera vaccine supply has now increased sufficiently to allow the resumption of life-saving preventive campaigns for the first time in more than three years.
Mozambique is the first country to restart vaccination, after the halt in 2022 caused by the global surge in cholera cases that escalated demand and led to chronic shortages of oral cholera vaccine (OCV) stocks.
The preventive vaccine campaign begins amid an ongoing cholera outbreak and the aftermath of floods that affected more than 700 000 people and displaced many, Gavi reports. The floods have disrupted health systems and damaged water systems, further increasing the risk of waterborne diseases like cholera.
“Global vaccine shortages forced us into a cycle of reacting to cholera outbreaks instead of preventing them. We are now in a stronger position to break that cycle. I thank EUBiologics, currently the only manufacturer producing cholera vaccines at the scale needed for mass vaccination campaigns, for its efforts, and urge others to enter this vital space. These vaccines will save lives,” said Dr Tedros Adhanom Ghebreyesus, WHO Director-General.
A first allocation of 20m doses is being deployed for preventive campaigns. Of these, 3.6m doses were delivered to Mozambique; 6.1m to the Democratic Republic of the Congo that is also experiencing significant outbreaks; and 10.3m doses are planned for delivery to Bangladesh.
After sustained efforts by global agencies, manufacturers and partners, annual global supply of OCV has doubled from 35m doses in 2022 to nearly 70m doses in 2025. The doses are being financed by Gavi, and procured and delivered to countries by Unicef.
“The multi-year surge in cholera cases and resulting unprecedented demand for vaccines were stark reminders that sustainable, accessible vaccine supply is a global public good – and the world cannot afford complacency,” said Dr Sania Nishtar, CEO of Gavi, the Vaccine Alliance.
The three countries were chosen based on allocation criteria set out by the Global Task Force for Cholera Control (GTFCC), a partnership of more than 50 organisations, to ensure cholera vaccines for preventive campaigns are distributed systematically, equitably and transparently.
The OCV is safe, effective and recommended for individuals over 12 months old. One dose provides short-term protection for at least six months and can help bring outbreaks under control, while two doses provide protection against infection for longer – at least three years.
While global vaccine supply steadily improves, the one-dose strategy will remain the standard for outbreak responses, with the use of two doses considered on a case-by-case basis.
More than 600 000 cases of cholera, or acute watery diarrhoea, and nearly 7 600 deaths were reported to WHO from 33 countries last year, although these are under-estimates as cholera remains under-reported.
Since 2021, global cholera cases have risen year after year, with a decline observed in 2025. Cholera deaths, however, have continued to increase over the same period.
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WHO shifts to one-dose cholera jab as outbreaks increase, stocks dwindle
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