HPV Vaccine Dosing Debate: Could One Shot Replace Two?

Medical experts are weighing whether one dose of the HPV vaccine, rather than two, may be enough to protect preteens from cancers caused by the virus. The discussion marks a potential shift in guidance that has stood for years, even as U.S. vaccination recommendations have grown increasingly fractured.

HPV, or human papillomavirus, is the most common sexually transmitted infection in the United States. More than 40 types can infect the genital areas of men and women as well as the lining of the mouth and throat. Certain strains cause most genital warts, and the virus is linked to a large share of the roughly 50,000 cancer cases diagnosed annually in those parts of the body. About 90% of cervical and anal cancers are attributed to HPV, according to the Centers for Disease Control and Prevention.

The first vaccine, Merck & Co.'s Gardasil, was approved in 2006, followed by GlaxoSmithKline's Cervarix in 2009. Both are cleared for males and females from age 9 into the 20s, but were prioritized for 11- and 12-year-olds because the shots work best before sexual activity begins.

Initially recommended only for girls as a three-dose series over six months, the regimen was extended to boys in 2011, largely to prevent them from spreading a virus that can cause cervical cancer in girls. A decade ago, the government cut the recommended doses from three to two after studies showed sufficient protection, with research suggesting the two doses worked better when spaced six to 12 months apart.

Since then, studies across multiple countries have recorded sharp declines in infections, genital warts and precancerous cervical lesions. In 2020, the American Cancer Society raised the age at which women should begin screenings to 25 from 21, citing data indicating vaccinations had reduced rates of cervical changes that precede cancer.

More recent evidence has been favorable. The University of Minnesota's Vaccine Integrity Project published a review this year finding that HPV vaccines lowered the risk of genital-area cancers in women. A study in the journal JAMA Oncology in April found vaccinated males were nearly 50% less likely to develop cancers of the head and neck, esophagus, anus or penis. "The duration of protection is long," said Angela Ulrich, who led the review, noting that people in their 20s, 30s and 40s are still seeing protection.

A recent CDC report found HPV vaccination rates lag behind some other adolescent shots, partly because most states do not require the vaccine for school attendance. The rate has been flat for four years, with about 63% of teenagers receiving both doses. The shots are also known for a sharper sting, and doctors have noted fainting is more common after them. Dr. Scott Hadland, who works at a Boston clinic treating adolescents and young adults, said some patients report more pain than other vaccines, though only briefly.

The debate has also been shaped by politics. Health Secretary Robert F. Kennedy Jr., before taking office last year, was an attorney and consultant in lawsuits against Merck over Gardasil and falsely claimed the vaccine increases cervical cancer risk. In January, he ordered the recommended HPV doses reduced to one as part of a unilateral cut to the childhood vaccination schedule. A federal judge blocked the change.

Yet the idea had been under expert consideration for some time. It gained attention late last year with a Costa Rica study that tracked more than 20,000 girls over five years and found a single shot provided about 97% protection against cervical cancer, similar to two doses. In 2024, a CDC advisory committee formed a work group to explore a revised recommendation, but the effort was interrupted when Kennedy dismissed the committee's members.

The American Academy of Pediatrics, one of the groups that sued Kennedy, sets its own recommendations and has been considering a guidance change, examining whether one dose is sufficient for some people and at what age vaccination should begin, said Dr. James Campbell, who chairs its committee on infectious diseases.

For now, experts continue to advise two shots. While evidence suggests one dose was sufficient for girls against cervical cancer, there is less evidence regarding other cancers in girls and whether a single shot protects boys against the HPV-related cancers they face, according to the Vaccine Integrity Project review. Whatever the recommendation, Hadland said, the priority is that children get vaccinated. "The benefits are very clear. It prevents cancer," he said.