Elizabeth Renstrom for TIME
By Alexandra Sifferlin
April 21, 2015
TIME Health
For more, visit TIME Health.

A new study underlines the effectiveness of the HPV vaccine, showing the vaccine is protective against the virus on multiple sites on the body, even for women who have been infected in the past.

In a randomized controlled trial—considered the gold standard of scientific research—scientists wanted to know if the HPV vaccine protected against cervical, anal and oral HPV. Daniel C. Beachler, a postdoctoral fellow in the Infections and Immunoepidemiology Branch of the National Cancer Institute (NCI), and his colleagues followed 4,186 women between the ages of 18 to 25 who were either vaccinated with a HPV16/18 vaccine or a control vaccine (a hepatitis A vaccine). Cervical samples from the women were collected at their annual visits and oral and anal samples were collected at a four-year follow-up visit.

“We were interested in the question of whether the vaccination may protect non-infected sites against HPV infection or re-infection in women who were previously exposed to HPV prior to vaccination,” says Beachler. The study was presented at the American Association for Cancer Research annual meeting.

The results showed that the efficacy for the vaccine in all three sites was 83% among the women with no evidence of prior HPV exposure and infection, 58% among women with prior HPV exposure, and a 25% among women with active cervical HPV16/18 infection (the percentage was considered nonsignificant). In total, the researchers report that the overall vaccine efficacy was 65% for all sites and 91% for protection in at least two sites.

Among the women in the trial, some had no evidence of HPV, some had an active HPV infection, and some of the women did not have an active infection but had antibodies for HPV, suggesting that they had been exposed to the virus previously. That’s not uncommon, considering the U.S. Centers for Disease Control and Prevention (CDC) estimates that nearly all sexually active men and women get HPV at some point in their lives.

The CDC says teen boys and girls who did not start or finish the HPV vaccine series when they were younger should get it, and young women can get the vaccine through age 26 and young men through age 21.

The CDC says that girls who are already sexually active can still benefit from the vaccine, but it may be less effective since it’s possible they have already been exposed to one or more HPV strains. Still, the CDC says that since young women are not necessarily infected with all types of HPV, they can still benefit from the vaccine. This new study underscores that guidance.

“[This study] is supportive that there could be some benefit at these older ages,” says Beachler. “Close to 90% of individuals are able to clear an HPV infection on their own. This is not a therapeutic vaccine but it could still help protect from acquisition of new infections.”

Dr. Miriam Lango, a head and neck cancer surgeon at Fox Chase Cancer Center, says that the new study is some of the “best kind of evidence we have,” in support of vaccinating against HPV in women of that age. “My understanding was always that you get the vaccination before you get the infection and that after you’ve been infected there’s no benefit to having the vaccination,” she says. “That’s really not what the data tells us.” Lango was not involved in the study.

Beachler noted that at the end of the study, the women in the control arm of the trial were able to get vaccinated against HPV.

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