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A new study seems to confirm that a single dose of the vaccine used to prevent HPV infection is just as effective as two — and, therefore, also helps to prevent cancer.
CHICAGO — A clinical trial run by the National Cancer Institute seems to confirm that a single dose of the vaccine used to prevent infection with the human papilloma virus is just as effective as two — and, therefore, also helps to prevent cancer. The result could transform efforts to reach the three-quarters of children globally who should receive the vaccines but don’t. The shots prevent cervical cancer and also anal, penile, and some head-and-neck cancers. Worldwide, 350,000 women die from cervical cancer, the most common HPV cancer. “I think we can all agree that one dose and two doses of … these fantastic vaccines work really well,” said Aimée R. Kreimer, the NCI investigator who presented the new data here Monday at the annual meeting of the American Association for Cancer Research. “We’re really excited about these results.” In developed countries, including the U.S., two doses of the vaccines are currently recommended. But in the clinical trial, whether one dose or two doses were used, the results were equivalent, according to the study of 20,000 girls between ages 12 and 16 in Costa Rica who were followed for 4 1/2 years. In all cases, the estimated efficacy of the vaccines was upwards of 97%.
Experts were fast to praise the results. Chyke A. Doubeni, chief health equity officer The Ohio State University Wexner Medical Center, called them “phenomenal” from the conference stage. He grew up in Nigeria and said he could imagine the findings could help expand access to more people in developing countries. Norman “Ned” Sharpless, who was NCI director in the first Trump administration and is now a managing director at Jupiter Bioventures, echoed those comments. “I think this is the most important cancer prevention trial going on in the world. We have a long history of failure in cancer prevention,” Sharpless said. “To have such clear evidence to support our practice is terrific and this will make the dissemination of HPV vaccine worldwide much easier and much cheaper to do — and it will save millions of lives.”
The results could prove controversial not because of science but U.S. politics. Robert F. Kennedy Jr., the secretary of the Department of Health and Human Services, has long criticized HPV vaccines, particularly Gardasil, made by Merck, which is the HPV vaccine used in the U.S. Kennedy has asserted on social media that Gardasil may increase the risk of cervical cancer, a claim contradicted by many studies. During his Senate confirmation hearing, he refused to disavow previous comments he had made questioning the safety of the vaccine.
At Children’s Health Defense, the nonprofit group where he served as founder and chairman for years, Kennedy filed more than two dozen lawsuits over the vaccine. During the nomination process, he promised to give any fees he earned from the Gardasil litigation to his son. But from the start, the development of the HPV vaccines and studies of them could be traced back to the NCI, which is part of HHS. The institute’s deputy director, currently its highest-ranking official, is Douglas Lowy, who was a key figure in the development of the vaccines. He has won multiple scientific prizes, including the National Medal of Technology and Innovation and the Lasker-DeBakey Clinical Medical Research Award, for his work. The NCI has also spent decades studying how to increase the uptake of hepatitis B and HPV vaccines as a way of preventing cancer. In developed countries, the HPV vaccines have had a dramatic impact. In the U.S., Kreimer said, they have decreased the incidence of precancerous cervical lesions by 80% — particularly impressive because only 77% of people in the U.S. who should receive the vaccine have been administered at least one dose. In the U.S. and other developed countries, effective screening against these lesions means that other types of HPV cancer, in particular head-and-neck cancer, are as common as cervical cancer. This is one reason vaccination is recommended for boys as well as girls.
Studies across the world have affirmed that HPV vaccination does prevent cancer. In one stunning recent example, Scotland said in 2024 that, of girls in the country who were born between 1988-1996 and had been vaccinated in time, none had been diagnosed with cervical cancer. Australia has said that it believes it can eliminate cervical cancer through vaccination and screening.
In the United States, giving two shots of the vaccine has been difficult to achieve on a wide scale among adolescents — a reality that cancer prevention experts have long lamented. In 2019, only 54.2% of adolescents had received two shots of an HPV vaccination. But these new data suggest that the picture is perhaps not as dire as originally feared. In less developed countries, screening for cervical pre-cancers is far less common. These have always been the places where the HPV vaccines could have the biggest impact — but, because the vaccines are costly and because having to give multiple shots of a vaccine makes rolling it out more difficult in these countries — the vaccines have had the least impact there. In her talk, Kreimer said that only 24% of the girls who should get the vaccine worldwide do. That’s why showing that one dose of the vaccines is as effective as two could be so important. And much of this important work has come from the NCI. Rolando Herrero, one of the researchers involved in the new study, was the lead investigator of a study that in 2007 first raised the prospect that fewer doses of an HPV vaccine could be just as effective. That study, the Costa Rica Vaccine Trial, tested three doses of the vaccine Cervarix, which was the dosage used at the time. But 20% of the volunteers in the study received fewer than three doses, in part because there were contraindications to receiving other doses — including becoming pregnant. It appeared that those who received only one dose were protected from HPV just as well as those who were fully vaccinated. “This flew in the face of vaccine dogma,” Kreimer said. At the time, researchers thought protein-based vaccines like Cervarix always needed at least two doses to result in a durable result. But the finding led to other research, including a study in India where a trial stoppage meant that many patients only received one dose of the vaccine. The World Health Organization moved to recommenda one-dose HPV vaccine course. The U.K. also moved to recommend giving just one dose in 2023.
There will not be much debate about using one dose in countries where vaccination has proved difficult. Kathrin Jansen, who led development of Gardasil at Merck and is now retired, noted that in a setting where people are not vaccinated one dose is certainly better than none. “I don’t think there’s going to be any finding in the recent past that’s going to democratize cervical cancer control as much as these studies have,” said CS Pramesh, of the Tata Memorial Centre in Mumbai, from the AACR stage. But there is a question of how quickly the U.S. will consider moving to a single-dose regimen. There are still gaps in the data. For instance, researchers could want more follow-up on vaccine durability. And there are no data on other types of cancers HPV causes, particularly with regard to cancers of the mouth and throat. Still, in recent deliberations, the U.S. Advisory Committee on Immunization Practices has seemed open to a change. On Merck’s recent earnings calls, the company’s chief scientific officer, Dean Li, pushed back against the idea that such a move would come quickly. “I would just emphasize: we have had clear , firm, consistent, and recent guidance from the FDA on what [they] would require for the licensure of a reduced dosing or a single dose. They are very clear on the high evidentiary standard,” Li said, according to a transcript. He added: “The FDA is incredibly up-to-date and aware of the limitations of existing trials, and none meet the criteria for them to have a label change.” Reached for comment, a Merck spokesperson reiterated Li’s statement about the FDA’s high standard. “The FDA is aware of the limitations of ongoing and existing trials, and based on what we know currently, none meet the criteria for approval of a label change to reduced dosing,” the spokesperson said. Kreimer pointed out that the benefit of change could be measured in lives saved.
“Cervical cancer is a horrible disease, and outside the U.S., in lower-income countries, it is a death sentence the incidence is the mortality,” she said. Better then, to keep the cancer from ever happening in the first place.
