Health experts are reevaluating the recommended number of HPV vaccine doses for adolescents, as recent research suggests a single shot might offer protection similar to two doses. The Centers for Disease Control and Prevention (CDC) states that about 90% of cervical and anal cancers are linked to HPV infections. HPV, or human papillomavirus, is the most common sexually transmitted infection in the U.S., with over 40 types that can infect genital areas and the lining of the mouth and throat. Certain strains are responsible for most genital warts and are a leading cause of nearly 50,000 cancer cases diagnosed annually in the U.S. The first HPV vaccine, Merck & Co.'s Gardasil, was approved in 2006, followed by GlaxoSmithKline’s Cervarix in 2009. Both are approved for use in males and females, available for children as young as 9 and adults in their 20s. Initially, HPV vaccinations were recommended only for girls and given as a series of three shots over six months. Boys were added in 2011, largely to prevent them from spreading the virus to girls, which could cause cervical cancer. In 2010, the dosing recommendations were changed from three to two shots after studies showed that amount offered sufficient protection. Research suggested two doses worked better when spaced six to 12 months apart. Studies in multiple countries have shown significant decreases in HPV infections, genital warts, and precancerous cervical lesions. In 2020, the American Cancer Society updated its guidelines for women, raising the starting age for cervical cancer screenings to 25 from 21, citing data indicating vaccinations had led to a drop in rates of cervical changes that are precursors to cancer. A research review by the University of Minnesota's Vaccine Integrity Project found that HPV vaccines had lowered the risk of genital-area cancers in women. 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. Despite these benefits, HPV vaccination rates remain lower than for other vaccines, partly because most states do not require HPV vaccinations as a condition of school attendance. A recent CDC report found vaccination rates against HPV were lower than for some other shots adolescents get, with about 63% of teenagers getting both doses. Some adolescents report discomfort with the shots, with doctors noting fainting is more common after HPV shots. Dr. Scott Hadland, who works at a Boston clinic treating adolescents and young adults, said some patients report the shots hurt more than other vaccines they've received, but only for a moment. In January, Robert F. Kennedy Jr., who was appointed U.S. health secretary last year, 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. However, the idea of reducing the number of doses had been considered by medical experts for some time. A study in Costa Rica tracked over 20,000 girls over five years and found a single HPV shot provided about 97% protection against cervical cancer, similar to two doses. 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 and has been considering a guidance change, exploring whether one dose is sufficient for at least some people and at what age vaccination should begin. 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.