MIKE STOBBE NEW YORK — Preteens may soon have something to celebrate: Medical experts are discussing the possibility that one dose of a life-saving cancer vaccine, instead of two, may be enough to protect them.

U.S. vaccination recommendations have become splintered in the past year, with federal officials and doctors’ organizations disagreeing over guidance and even battling each other in court. But both sides have been exploring revisions in longstanding HPV vaccine recommendations.

“This would be a significant shift in terms of what has been communicated for many years about the best way to protect adolescents against HPV,” said Jason Schwartz, a Yale University vaccine policy researcher.

Here’s a look at HPV shots, which debuted 20 years ago and may now be approaching a crossroads. HPV, human papillomavirus, is the most common sexually transmitted infection in the U.S.

More than 40 HPV types can infect the genital areas of men and women, plus the lining of the mouth and throat. Certain strains are responsible for most cases of genital warts.

HPV also is a leading cause of the nearly 50,000 cancer cases diagnosed in the U.S. each year that affect those parts of the body. Indeed, about 90% of cervical cancer and anal cancers are attributed to HPV infections , according to the Centers for Disease Control and Prevention.

In 2006, federal health officials approved the first HPV vaccine, Merck & Co.'s Gardasil. A second vaccine was approved in 2009, GlaxoSmithKline’s Cervarix.

Both vaccines are approved for use in males and females, available for kids as young as 9 to adults in their 20s. But they were prioritized for 11- and 12-year-olds, because the shots work best if given before someone is sexually active.

Initially, in 2006, HPV vaccinations were recommended only for girls and given as a series of three shots over six months. Boys were added in 2011, a move largely intended to stop them from spreading a virus to girls that could cause cervical cancer.

Ten years ago, the government changed the dosing recommendations. It dropped the number of recommended doses from three to two after studies showed that amount offered sufficient protection. Research also suggested two doses worked better when spaced six to 12 months apart, meaning they could be given at annual checkups.

Since then, studies in multiple countries have shown dramatic decreases in infections, genital warts cases and precancerous cervical lesions.

In 2020, the American Cancer Society updated its guidelines for women, raising the age at which screenings should start to 25, from 21. The organization attributed the change to data indicating vaccinations had led to a drop in rates of cervical changes that are seen as precursors to cancer. Evidence has continued to be favorable.

The University of Minnesota’s Vaccine Integrity Project this year published a research review that found that HPV vaccines had lowered the risk of genital-area cancers in women. And a study published in April in the journal JAMA Oncology found vaccinated males are nearly 50% less likely to develop cancers of the head and neck, esophagus, anus or penis.

What’s more, “the duration of protection is long,” said Angela Ulrich, who led the Vaccine Integrity Project review. “We have people who are 20, 30, 40 years old who are seeing protection from the HPV vaccine.” A recent CDC report found vaccination rates against HPV were lower than the rates for some other shots adolescents get, at least partly because most states don’t require HPV vaccinations as a condition of school attendance.

Still, the HPV vaccination rate has been flat for four years in a row, with about 63% of teenagers getting both doses.

It doesn’t help that HPV shots are known to have a sharper sting. (Doctors have noted fainting is more common after HPV shots.) Questions about pain are common, said Dr. Scott Hadland, who works at a Boston clinic that treats adolescents and young adults. “Some of my patients tell me it hurts more than other vaccines they’ve received, but only for a moment,” he said.

Also not helping lately, perhaps, is the elevation of Robert F. Kennedy Jr. to U.S. health secretary. Before he was appointed to oversee federal health agencies last year, Kennedy was an attorney and consultant in lawsuits against Merck over the Gardasil vaccine, and falsely claimed the vaccine increases cervical cancer risk.

Also not helping lately, perhaps, is the elevation of Robert F. Kennedy Jr. to U.S. health secretary. Before he was appointed to oversee federal health agencies last year, Kennedy was an attorney and consultant in lawsuits against Merck over the Gardasil vaccine, and falsely claimed the vaccine increases cervical cancer risk.

In January, Kennedy ordered the number of recommended HPV doses to be reduced to one, as part of a unilateral reduction of the childhood vaccination schedule. That change was blocked by a federal judge’s order, but it actually is something that medical experts had been considering for some time.

That change was blocked by a federal judge’s order, but it actually is something that medical experts had been considering for some time.

The idea gained attention late last year with the publication of a study in Costa Rica that tracked more than 20,000 girls over five years. It found a single HPV shot provided about 97% protection against cervical cancer, similar to two doses.

A year earlier, in 2024, research developments prompted a CDC advisory committee to form a work group to explore the possibility of a revised recommendation. That work was interrupted when Kennedy dismissed the members of the committee.

The American Academy of Pediatrics — one of the medical groups that sued Kennedy — makes its own vaccination recommendations. It has been considering a guidance change, exploring questions such as whether one dose is sufficient for at least some people and at what age vaccination should begin, said Dr. James Campbell, chair of the organization’s committee on infectious diseases. Recommendations are evolving. But right now, experts advise that kids get two shots.

While there is evidence showing one shot was sufficient for girls in preventing cervical cancer, there is less evidence about other kinds of cancer in girls. Also lacking is data on whether one shot is sufficient protection for boys against the HPV-related cancers they are at risk for, according to the Vaccine Integrity Project review.

Whether the recommendation remains two shots or is changed to one, it’s important that kids get vaccinated, Hadland said. “The benefits are very clear. It prevents cancer,” he said.

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