The results of large-scale study suggest that one shot of the HPV vaccine may be enough to protect young people from HPV and related issues like cervical cancer. The results were presented at the meeting of the American Cancer Society and are being discussed by the Advisory Committee on Immunization Practices (ACIP).

When the HPV vaccine was first introduced in 2006, expert believed that three doses were needed to get the benefits of immunity. Young people who got the first shot would get a second dose one a month later and a third shot six months after that. This schedule was hard for some young people and their parents to keep up with, and some people did not receive all three recommended doses.

Over the last twenty years, there has been a lot of research suggesting that three shots were not necessary. Early research showed that two shots provided a similar level of protection, especially in younger people who are known to have stronger immune reactions. In 2016, the ACIP changed its recommendations and began saying that anyone who got their first shot before the age of 15, only needed two shots. This new study may lead to yet another change in the vaccination schedule.

HPV Vaccine Study—One Shot Compared to Two

The ESCUDDO trial enrolled over 20,000 girls ages 12-15 in Costa Rica beginning 2017. Girls were randomly assigned to one of four groups. The first group got one dose of Cervarix, a vaccine that protects against the two types of HPV most likely to cause cervical cancer. The second group got two doses of Cervarix. (Cervarix is no longer available in the United States but is still used in other countries.) The third group got one dose of Gardasil 9, which protects against the 9 strains of HPV most likely to cause cancer or genital warts. The fourth group got two doses of Gardasil.

The young women were followed every six months. They were tested for HPV infection of the cervix. They were also given blood tests to see if they’d produced antibodies against the types of HPV in the vaccine. Researchers found no significant differences for either of these factors between those who got one shot and those who got two of either vaccine.

In addition to these test subject, the researchers enrolled a group of unvaccinated women ages 16-22 from the same geographic areas. These women attended three study visits six months apart to determine their HPV DNA status. For ethical reasons, they were offered HPV vaccination each time. Researchers compared these test results to all of the study groups to get a sense of how many potential cases of HPV were avoided because of the vaccines. They also looked at whether this was different in people who got one or two doses of their vaccine. It was not.

The researchers concluded that for both vaccines, one dose was as good as two. This study confirms the findings of similar studies in other parts of the world. A 2016 study from India, for example, found that one dose of an earlier version of Gardasil provided sufficient protection. A similar randomized trial from Kenya found that one dose of Gardasil 9 or Cervarix was 98% effective in preventing HPV 16/18 infection up to three years post-vaccination.

What a Change to One Shot Could Mean

Moving to a single dose of the HPV vaccine could help increase vaccine uptake by eliminating some of the barriers (like transportation and time off from work) that prevent some people from returning to a health care provider for additional doses. It will also make large-scale vaccination efforts easier and more affordable especially in low resource countries.

The ACIP discussed one-dose of the HPV vaccine at its last meeting in April and may decide to change its recommendations when it meets again in June. This may be complicated, however, by anti-vaccine sentiment within the Department of Health and Human Services and funding cuts to the CDC.