Carissa Ray / msnbc.com The HPV vaccine doesn't encourage girls to have sex, a new study says.

The HPV vaccine does not send teenage girls out seeking sex, contrary to the protests of some parents who worried about immunizing young girls against a sexually transmitted virus, researchers reported Monday.

While several surveys of teen girls suggested they would not feel free to have sex just because they’d been vaccinated against the virus, this is the first one to make sure that girls were not, in fact, more likely to engage in sexual activity after vaccination.

“We wanted to do an objective clinical survey using data,” said Robert Bednarczyk of the Kaiser Permanente Center for Health Research and Emory University in Atlanta, who led the study.

The human papillomavirus, or HPV, is the main cause of cervical cancer, as well as genital warts. It can also cause other cancers, including cancer of the mouth, head and neck, penis and anus. The vaccine protects against the main cancer-causing strains.

“In 2006, the Advisory Committee on Immunization Practices recommended that all US girls aged 11 to 12 receive the human papillomavirus (HPV) vaccine, with catch-up vaccination recommended through age 26, and administration permitted as young as 9 years,” Bednarczyk’s team wrote in their report, published in the journal Pediatrics.

Researchers say HPV is behind the increased rates of throat cancer in men. WKYC's Monica Robins reports.

Many parents were dubious about vaccinating such young girls against a sexually transmitted disease. A few, and some religious groups, said the vaccine would even encourage sexual activity.

But HPV is extremely common. The Centers for Disease Control and Prevention estimates that 20 million Americans are currently infected with HPV. Another 6 million people become newly infected each year. For most, the virus clears their system on its own, but at least 50 percent of sexually active men and women are infected at some point in their lives. Some estimates range as high as 80 percent.

“About one-third of 14- to 19-year-olds are positive for at least one HPV strain,” Bednarczyk said. And a study published in August found some girls were infected even if they'd never had sex.

NBC's chief medical editor, Dr. Nancy Snyderman, and gynecologist Dr. Suzanne Gilberg-Lenz discuss the Gardasil vaccine, which is designed to protect women from strains of HPV that can cause cervical cancer.

Cervical cancer kills 3,870 women a year in the United States and 300,000 globally. There are two commercial vaccines -- Merck’s Gardasil and its rival, GlaxoSmithKline’s Cervarix. The idea behind starting to vaccinate 11- and 12-year-olds is to allow full immunity to build long before they ever have any kind of sexual contact for the first time.

It is possible that girls might be confused about the protection provided by the vaccine, Bednarczyk said. They might mistakenly believe it protects them against all sexually transmitted diseases, or even against pregnancy. So he sought cold, hard data on what girls actually did after they got vaccinated.

Bednarczyk and colleagues went through the medical records of nearly 1,400 girls who got either the HPV or another immunization such as the meningitis vaccine. They followed them for three years after vaccination.

“We looked for the first occurrence of any testing or diagnoses related to pregnancy or sexually transmitted infections or any counseling on contraceptive use,” Bednarczyk said in a telephone interview.

“We got up to age 16 which, in some surveys, is where you start seeing more sexual activity among adolescents.”

There were no differences between girls who got HPV vaccine and girls who got other vaccines but not HPV, the team found. “Overall, what we found through the whole follow-up study was among 1,400 girls only eight actual cases of either pregnancy or sexually transmitted infection,” Bednarczyk said.

The rates were identical in the HPV vaccine and non-HPV-vaccine groups. “We feel this is reassuring,” he said. “We can start to move beyond these concerns.”

Rates of sexual activity are going down among school-aged children. The CDC says while 39 percent of all 15- to 17-year-olds had had sex in 1995, the rate fell to 27 percent by 2010. But around 3 percent of girls report they started having sex before the age of 13.

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