In 1996, Guinea-Bissau seemed like an ideal research post for budding pediatrician Lone Graff Stensballe. Her supervisor, a fellow Dane named Peter Aaby, had spent nearly two decades collecting data on 100,000 people living in the mud brick homes of the West African country’s capital.

Aaby and his partner, Christine Stabell Benn, believed that the years of research in the impoverished country had yielded a major discovery about vaccines—and what they described as “non-specific effects”: The measles and tuberculosis vaccines, which were derived from live, weakened viruses and bacteria, they said, boosted child survival beyond protecting against those particular pathogens.

But, the scientists said, shots made from deactivated whole germs, or pieces of them, such as the diphtheria-tetanus-pertussis (DTP) shot, caused more deaths—especially in little girls—than getting no vaccine at all.

The World Health Organization repeatedly and inconclusively examined these astonishing findings. They tended to elicit shrugs from other global health researchers, who found Aaby’s research techniques unusual and his results generally impossible to replicate.

Then came Donald Trump, Covid, and the administrative reign of anti-vaccine advocate Robert F. Kennedy Jr.

Suddenly, Aaby and Benn weren’t just sending up distant smoke signals from a far corner of the planet. They were confidently voicing their views and policy prescriptions online and in medical journals. The “framework” for “testing, approving, and regulating vaccines needs to be updated to accommodate non-specific effects,” their team wrote in a 2023 review.

And the Trump administration has taken notice.

“They became more strident in saying that their findings were real and that the world needed to do something about it,” said Kathryn Edwards, a Vanderbilt University vaccinologist who has been aware of Aaby’s work since the 1990s. “And they became more aligned with RFK.”

Kennedy, as secretary of the Department of Health and Human Services, cited one of Aaby’s papers to justify slashing $2.6 billion in US support for Gavi, a global alliance of vaccination initiatives. The cut could result in 1.2 million preventable deaths over five years in the world’s poorest countries, the nonprofit agency has estimated. Kennedy has frozen $600 million in current Gavi funding over largely debunked vaccine safety claims.

Kennedy described the 2017 paper as a “landmark study” by “five highly regarded mainstream vaccine experts” that found that girls who received a diphtheria-tetanus-pertussis, or DTP, shot were 10 times more likely to die from all causes than unvaccinated children.

In fact, the study was far too small to confidently make such assertions, as Benn acknowledged. In a study of historical data that included 535 girls, four of those vaccinated against DTP in a three-month period of infancy died of unrelated causes, while one unvaccinated girl died during that period. A follow-up published by the same group in 2022 found that the DTP shot by itself had no effect on mortality. Critics say the 2017 study, rather than being a landmark, exemplified the troubling shortfalls they perceive in the Danish team’s research.

As Aaby and Benn’s US profile has risen, scientists in Denmark have set upon the work of their compatriots. In news and journal articles published over the past 18 months, Danish statisticians and infectious disease experts have said the duo’s methods were unorthodox, even shoddy, and were structured to support preconceived views. A national scientific board is investigating their work.

Stensballe, who worked with Aaby and Benn for 20 years, has been among those voicing doubts.

“It took years to see what I see clearly today, that there is a strange concerning pattern in their work,” Stensballe said in a phone interview from Copenhagen, where she treats children at Rigshospitalet, the city’s largest teaching hospital. She said their work is full of confirmation bias—favoring interpretations that fit their hypotheses.

Those hypotheses overlap, in important areas, with the notions of Kennedy and other vaccine-skeptical officials at HHS.

In December, HHS announced the agency would award the Bandim group $1.6 million to study whether the birth dose of hepatitis B vaccine weakens babies’ immune systems or causes neurological issues.

The researchers plan to withhold the vaccine from half of the 14,000 newborns in the study, although the long-established vaccine is 90 percent effective in preventing infection. The Bandim group justifies this decision by noting that impoverished Guinea-Bissau does not yet routinely vaccinate infants against hepatitis B. Given that 1 in 5 Guinea-Bissauan adults carry the hepatitis B virus, however, the WHO and many infectious disease specialists say it is unethical to withhold the birth dose.

Aaby and Benn did not respond to repeated requests for comment. They have vigorously defended their work elsewhere.

A Mixed Reputation

Many Danes admire the two for their decades of work in Guinea-Bissau, a nation of over 2 million people where, as in much of Africa, infant mortality has plunged over the past five decades. There’s even a novel, the 2013 Danish thriller The Arc of the Swallow, featuring a corporate plot to murder a scientist character clearly based on Aaby. The company’s goal: to keep him from publishing data showing deadly effects from the DTP shot. Benn has said she gave the author the idea for the book.

Aaby and Benn have trained around 50 scientists through their Bandim Health Project, named for a district of Bissau, Guinea-Bissau’s capital. The research group has published over 1,000 academic papers and won scientific prizes. The Danish king knighted Benn last year. Their notion of non-specific vaccine effects gained enough traction to merit a short chapter in the 2023 edition of Plotkin’s Vaccines, the authoritative text of vaccinology.

Yet Danish health authorities have never followed Aaby and Benn’s vaccine advice. They still offer vaccines based on inactivated viruses and bacteria, on a schedule that Kennedy largely shifted the US to in January. (A federal judge on March 16 temporarily blocked those changes.) Danish vaccine authorities are considering the addition of two of the shots Kennedy sought to drop from the US schedule—against rotavirus and respiratory syncytial virus, or RSV.

“What’s important is that Christine doesn’t have influence on our vaccine policy,” said Anders Hviid, chief epidemiologist at Statens Serum Institut, the Danish equivalent of the Centers for Disease Control and Prevention.

Hviid—who knows Benn, as do most members of the tiny Danish vaccine fraternity—has contributed to many vaccine safety studies, including a 2019 paper that found no link between measles-mumps-rubella, or MMR, vaccination and autism. Kennedy tried unsuccessfully to get a journal to retract a recent Hviid study showing no link between aluminum-adsorbed vaccines and allergies or neurodevelopmental disorders.

In a 2023 podcast with Tracy Beth Høeg, the Danish American sports medicine doctor and Covid vaccine skeptic who led the FDA’s drug regulation from December until she was fired on May 15, Benn said she had vaccinated her son and daughter, now in their late twenties, under the complete Danish schedule of vaccines. Like the US schedule, Denmark’s includes a less reactive form of the DTP shot known as DTaP.

These vaccines aren’t dangerous to kids in well-off countries like the US and Denmark, she said. But she said she would “never vaccinate my child according to the US program.” She singled out the hepatitis B vaccine birth dose, which her group plans to test in Guinea-Bissau, saying she was “appalled” the CDC recommended a universal birth dose.

RFK Jr.’s hand-picked vaccine advisory committee—which a federal judge in March cast into limbo, questioning its members’ qualifications—withdrew the birth dose recommendation last year.

Compatriots Grow Skeptical

Kennedy’s championing of Aaby and Benn prompted criticism from Danish scientists that has extended to the ethics of the hepatitis B study. “It is disturbing that Danish researchers could carry out such actions involving African children,” Stensballe said.

As of early May, the study was paused while officials from Guinea-Bissau and the African Centers for Disease Control examined it. Public Health minister Quinhin Nantote, who took office after a November coup in Guinea-Bissau, said in January he had no evidence that the six-member ethics committee that signed off on the study earlier had ever met to discuss it. The University of Southern Denmark, where Benn teaches, also raised ethical concerns about the study.

HHS spokesperson Andrew Nixon told KFF Health News the proposed study was “based on the highest scientific and ethical standards” and “represents the world’s first and perhaps only opportunity to test the overall health effects” of the hepatitis B vaccine.

It’s only one area of the couples’ research that is under scrutiny.

In 2024, Danish physician and journalist Charlotte Strøm published an article noting that the Bandim group scientists had failed to publish data they’d collected that contradicted their frequent claims that the vaccine caused high mortality in infants.

Strøm called it “an ethical and scientific scandal,” and it led to an investigative series by the news outlet Weekendavisen. In February, the University of Southern Denmark forwarded its probe into the duo’s possible withholding of DTP data to the Danish Agency for Higher Education’s Board on Research Misconduct.

In response to the Weekendavisen articles, Aaby and Benn pushed out a version of the study. They said they hadn’t sought to publish it earlier because one coauthor died and another left the project after getting pregnant.

“This is a bit fishy,” said Henrik Støvring, a statistician at the University of Southern Denmark and Aarhus University who coauthored with Strøm and others an analysis challenging the methodology of clinical trials conducted by Benn and Aaby.

In January, a paper by Hviid and three other Danish infectious disease researchers questioned whether Aaby and Benn had actually proved that vaccines had bad or good “non-specific effects” beyond preventing the diseases they were designed to counter.

Scholars also have questions about Aaby and Benn’s studies of the tuberculosis vaccine, BCG. The pair recently began a study in which babies received a second vaccination with the live bacterial vaccine, although a similar revaccination study they conducted some 15 years earlier was stopped after 18 babies who got the vaccine died, compared with four in the control group, during a four-month span.

The study was aimed at testing Aaby and Benn’s hypothesis that the alleged dangers of DTP vaccination could be ameliorated by a shot soon after with live BCG.

Although there is some evidence that BCG provides a systemic boost to infant immune systems, the WHO does not recommend a second BCG dose, Vanderbilt’s Edwards noted. “Given the suspicion engendered with this group, there should be heightened attention to this protocol, with meticulous review of their work in Africa by the African authorities,” she said.

The Big Controversy

Aaby and Benn’s most controversial position is their stance on DTP, one of the most widely provided vaccines in the world. True evidence of its harm would be vitally important. And experts argue that research by others has not supported Benn and Aaby’s thesis.

One big recent study, involving nearly 55,000 newborns in Ghana and Tanzania, found that both BCG and DTP vaccines enhanced the survival of babies. The authors of the paper submitted it to a journal and fought long and hard with Benn, who happened to be a peer reviewer. They eventually resubmitted the paper to another journal to get it published in 2022, said coauthor Emily Smith, an assistant professor of global health at the Milken Institute School of Public Health at George Washington University.

Benn’s approach “involves splitting up trial data a bunch of different ways using a bunch of different methods,” she said.

“If you split up the data” enough ways, she said, “you’re going to end up with maybe thinking you found something.”

Hviid said that Benn and Aaby continuously modify their hypotheses to fit new data even when the patterns they detect may have popped up by chance. Most of the footnotes in their studies and opinion pieces refer to their own work, he noted.

“They’ve been talking about their paradigm for years,” Hviid said. “But when you look at the numbers, it’s just a house of cards. There’s nothing there.”

To examine their many hypotheses about the interactions of vitamins and vaccines, “hundreds of thousands of African babies have been tested,” Stensballe said. “Is that ethical?”

Aaby and Benn asked the editors of the journal Vaccine to retract Strøm and Støvring’s paper. The request was denied.

The Danish Influence in America

The Bandim group’s influence on US policy has roots in the Covid pandemic, when Benn befriended Høeg, who had earned a PhD in epidemiology and public health from the University of Copenhagen in 2014 for a study of eye disease. In a series of YouTube broadcasts, they bonded over skepticism about Covid vaccines and lockdowns. Benn argued that mRNA vaccines were insufficiently studied and that Covid should be allowed to run its course among kids. Høeg landed an adjunct professorship at the University of Southern Denmark, where Benn holds a senior position, in April 2023.

Høeg did not respond to a question about whether she was involved in the CDC decision to fund Benn’s hepatitis B study. Benn and Aaby also received $1.8 million from the Pershing Square Foundation, cofounded by Bill Ackman, an ally of President Trump who is skeptical of the US vaccine schedule. Ackman did not respond to requests for comment.

Emails that Weekendavisen reporter Gunver Lystbæk Vestergård obtained from the University of Southern Denmark showed that Benn secured the grant after communicating with anti-vaccine CDC officials Lyn Redwood and around the time the agency’s Advisory Committee on Immunization Practices was preparing to stop recommending hepatitis B vaccination for US newborns.

Yet during a public debate with Støvring on December 4, Benn said news reports had dried up all funding for her research. “You have literally closed our field station,” she said.

Aaby’s History

An anthropologist by training, Aaby, 81, has cultivated the image of a persecuted Galileo, Hviid said, “with us in the role of the dogmatic clergy.”

Aaby wrote in 1998 that he was “exploring and making sense of the unknown” while most of his colleagues’ work was “trivial.” At the December debate, he said Støvring’s work was “incredibly stupid.”

The Bandim Health Project’s study area covers six poor districts, now with about 200,000 inhabitants, around a third of the capital. The researchers say they have collected health and socioeconomic data from residents for more than 30 years.

Stensballe’s conflict with Benn and Aaby came to a head in 2015 as the team completed a study of 4,262 Danish babies comparing those who got a BCG vaccine at birth with those who didn’t. Aaby was certain that his African research on vaccines would be duplicated in the developed world.

The Danish BCG study showed no difference in hospitalization rates between the two groups. But Benn and Aaby combed the data for other answers, known as secondary findings, and leaped upon a comparison that showed lower hospitalization rates in babies whose mothers had been vaccinated against BCG decades earlier, Stensballe recalled.

She found that troubling. “If the primary outcome is negative, the trial is negative,” she said. In other words, while reworking data will inevitably find some positive result, a study’s conclusions should be based on the questions it was designed to answer.

The manner in which Aaby and Benn pose questions sows unnecessary doubt, said Arthur Reingold, a professor emeritus of epidemiology at the University of California-Berkeley.

“Some of the questions they propose to answer are important but can never be answered in my lifetime,” he said, “and not by an ethical study done in the real world.”

“And in the meantime,” he added, “babies will miss vaccines and get sick and die of preventable illness.”

KFF Health News is a national newsroom that produces in-depth journalism about health issues and is one of the core operating programs at KFF —the independent source for health policy research, polling, and journalism.