WEDNESDAY, Jan. 4 (HealthDay News) -- Nearly one in four girls who gets the human papillomavirus (HPV) vaccine mistakenly thinks that her risk of getting other sexually transmitted diseases is lowered, a new study indicates.

HPV is the most commonly transmitted sexually transmitted infection (STI) in the United States, with nearly 30 percent of sexually active girls aged 14 to 19 infected. Some virus types can raise the risk for genital warts and cervical cancer.

"I think it's important to counsel [girls] about what the vaccine protects against," said lead researcher Dr. Tanya Kowalczyk Mullins, an assistant professor of pediatrics at the Cincinnati Children's Hospital Medical Center and the University of Cincinnati College of Medicine.

The findings were reported in the January issue of the Archives of Pediatric and Adolescent Medicine.

Two HPV vaccines are now licensed in the United States. One, Gardasil, protects against two HPV strains linked with genital warts and two HPV types linked with cervical cancer. Another vaccine, Cervarix, induces immunity to the two HPV types linked with cervical cancer.

The HPV vaccine is recommended by the U.S. Centers for Disease Control and Prevention for boys and girls aged 11 to 12, with catch-up immunizations recommended to the age of 26 for women and 21 for men.

In the study, Mullins polled 339 girls, average age nearly 17, after their first of three HPV doses, and their mothers. Nearly 60 percent of the girls were sexually experienced.

Mullins wanted to know the girls' perceived risk of getting HPV after the vaccination, their perceived risk of getting other STIs and their perceived need for continued safer sex behaviors.

"Most girls correctly thought the vaccine does not protect them against STIs other than HPV," Mullins said.

However, 24 percent of the girls mistakenly thought they were at lower risk for other STIs such as syphilis and gonorrhea after the HPV vaccine, she found.

Girls who thought this were less likely to have information about the vaccine and about HPV infection.

Doctors who discuss the HPV vaccine with girls and their parents ''may need to emphasize the limitations of the vaccine and to specifically address that the vaccine does not prevent other STIs,'' the researchers wrote.

Mullins said it is not known how girls perceive risk after the entire three-dose series.

The study was supported by the U.S. National Institutes of Health.

Some study co-authors reported receiving grants from Merck, which makes Gardasil. One reported doing consulting work for Sanofi Pasteur, which has marketed Gardasil in Europe.

The study results bear "no big surprises," said Dr. H. Hunter Handsfield, a member of the board of directors for the American Social Health Association and a professor of medicine at the University of Washington Center for AIDS and STD in Seattle. He is an advisor to Merck for its HPV vaccine.

"If anything, it's a fairly pleasing result that it's only 24 percent [who think the HPV vaccine protects against other STIs]," he said.

Before the vaccines were available, Handsfield said, "social and religious conservatives" expressed worries that teens would practice safe sex less often after getting the vaccine. The latest findings suggest this isn't happening by and large, he noted.

Doctors do need to be clear with their young patients when giving them the HPV vaccine, Handsfield said. They need to tell them that the vaccine does not protect against all STIs, and that when the patients become sexually active they need to practice safer sex behaviors, such as using condoms.

More information

For more on HPV, go to the U.S. National Library of Medicine.