That news comes from a three-year, international study of more than 18,000 women aged 16-26. The study appears in The Lancet.
Gardasil appears to be "highly effective" against precancerous vulvar and vaginal lesions, especially in women who had never been exposed to the HPV strains that Gardasil targets, write the researchers.
They included Jorma Paavonen, MD, of the obstetrics and gynecology department at University Central Hospital in Helsinki, Finland.
Vulvar and vaginal cancer account for about 6% of gynecological cancers, and while less common than cervical cancer, cases of vulvar and vaginal cancer are rising. Surgery for those cancers can be mutilating, note the researchers.
HPV Vaccine Study
Paavonen and colleagues studied healthy young women in 24 countries in the Americas, Europe, and Asia. None of the women was pregnant.
The researchers gave all of the women three shots over six months. The women were randomly assigned to get Gardasil or an inactive shot (placebo).
The women got detailed physical checkups before receiving their first shot and again two months and six months later. After that, they got two checkups per year.
Since vulvar and vaginal cancers are relatively rare, the researchers checked the women for signs of precancerous vulvar and vaginal lesions linked to the HPV strains that Gardasil targets.
The HPV virus spreads through sexual contact. HPV usually doesn't cause cancer, but some strains are linked to the leading cause of cervical, vulvar, and vaginal cancer.
The researchers found that in women who hadn't been infected with two of the four HPV strains targeted by Gardasil, the vaccine was 97% to 100% effective against precancerous vulvar and vaginal lesions.
In women who had already been infected with those HPV strains when the study started, the vaccine was 71% effective against the same precancerous lesions.
"With time, such vaccination could result in reduced rates of HPV-related vulvar and vaginal cancers," write Paavonen and colleagues, calling the vaccine "highly effective" against the precancerous lesions.
They note that the vaccine would likely be most effective in women who were not yet sexually active. It's not known how long the vaccine's observed effects last or whether booster shots are needed, Paavonen's team notes.
The study was funded by Merck, which makes Gardasil. Several of the researchers work for or report financial ties to Merck.