A proposed trial of a hepatitis B vaccine on newborn babies in Guinea-Bissau has been cancelled following accusations that the trial was "unethical" and "neocolonial".
The trial was funded by the US, under Health Secretary Robert F Kennedy Jr, who has a long record of expressing scepticism about vaccines.
Police have seized a batch of vaccines from the research centre at the heart of the controversy.
The proposed trial drew international criticism because it planned to withhold a vaccine recommended by the World Health Organization (WHO) from half of the participating newborns.
Some public health experts argued that the proposed trial raised broader questions about whether vulnerable populations in African countries were being asked to take risks that researchers would not accept in wealthier countries.
The BBC has asked the US department of health and human services for comment.
Here is what you need to know.
The study was planned at the Bandim Health Project, a research centre that has operated in Guinea-Bissau since 1978.
It was designed to involve 14,000 newborns, with half receiving a hepatitis B vaccine shortly after birth and the other half not being given the shot.
Bandim said the trial aimed to investigate whether the hepatitis B vaccine given at birth has broader health effects beyond preventing infection.
Previous studies in The Gambia and Guinea-Bissau have raised questions about a possible link to increased mortality.
Researchers argued that because Guinea-Bissau does not currently offer routine hepatitis B vaccinations at birth, despite WHO recommendations, they have a unique opportunity to study its effects before such vaccines are introduced in 2028.
But critics questioned the ethics of withholding the vaccine from some babies, particularly in one of the world's poorest countries, where the infection is widespread.
The Trump administration funded the proposed study, with the CDC awarding the University of Southern Denmark (SDU) $1.6m (£1.2m) in December 2025.
Walter Kazadi Mulombo, the WHO representative in Guinea-Bissau, told the BBC they became concerned after receiving questions about the study and finding no publicly available protocol.
He said the main ethical concern was that the trial would withhold an intervention already proven to protect newborns against hepatitis B.
"The vaccine has been used for more than 30 years in more than 115 countries," he said. "A study that will withhold that intervention from other newborns is unethical."
He warned that babies could be exposed to the virus during childbirth if their mothers were infected, as well as later in life.
Nearly a fifth of the population of Guinea-Bissau are infected with hepatitis B, a viral infection that affects the liver.
Those infected during infancy are particularly likely to develop chronic infections, which kill up to 25% of people, mostly through liver failure or liver cancer, according to data published in 2020.
The WHO is working with the government to bring forward the birth-dose vaccine.
"The country had been expected to add it to its routine immunisation schedule in 2028, but officials are considering introducing it in the first quarter of next year," Mulombo said.
Guinea-Bissau currently vaccinates children against hepatitis B at six weeks, which means it is less effective.
Guinea-Bissau's government prohibited hepatitis B vaccine trials involving children in January. At the time, it requested a team of experts from the Africa Centres for Disease Control and Prevention (Africa CDC) to help evaluate the study.
Health Minister Quinhin Nantote said in October that the suspension remained in force and had not been overturned by the Council of Ministers after reports emerged claiming that the trial would be taking place after all.
"We were surprised to learn from media reports that the study might proceed," Nantote said. "We are outraged because even I, as the minister responsible for this sector, was not informed."
The dispute intensified after the SDU said a revised study protocol had received ethical approval "from the relevant authorities in Guinea-Bissau," which Nantote denied.
Dr Gavin Yamey, a professor of global health and public policy at Duke University in the US, criticised the proposal.
"The proposed trial reeks of neocolonialism," he told the BBC, questioning why the research was not being conducted in Denmark.
His criticism centred on the decision to withhold a recommended vaccine from some newborns in a country where hepatitis B is widespread.
SDU dean Jakob Grauslund said the trial had never started, no babies had been enrolled and no vaccinations administered as part of the research.
The proposed trial was funded by the US government at a time of significant changes to vaccine policy under President Donald Trump and Health Secretary Kennedy.
Under Kennedy's leadership, a US vaccine panel attempted to end a longstanding recommendation that all newborns receive a hepatitis B vaccine at birth, but a federal judge later blocked this, along with other changes to vaccine policy made by the Trump administration.
Critics have questioned whether the research could help support that policy shift, although the study's stated aim was to investigate potential effects of the vaccine beyond preventing infection.
Kennedy has praised the Danish researchers as "the deities of vaccine research", and cited earlier findings from their work in support of concerns about vaccine safety. A subsequent study by some of the same researchers, however, found no evidence of the previously reported effect.
There is no evidence in the information available that the proposed trial was designed to justify a change in US policy.
