August 15 (HealthDay News) -- Cervical cancer vaccines that prevent infection with the human papillomavirus (HPV) do not reduce or eliminate preexisting infections, a new study suggests.

That means that shots such as Gardasil, or a similar, yet-to-be-FDA-approved vaccine, Cervarix, should not be viewed as a treatment for women who've most likely contracted the highly common, sexually transmitted virus through their partners.

"From a public health perspective, a population-wide perspective, the best approach is to vaccinate girls and women before they initiate sexual activity," said study researcher Allan Hildesheim, senior investigator in the division of cancer epidemiology and genetics at the U.S. National Cancer Institute.

"You can then protect individuals prior to their being exposed, since the vaccine doesn't have any effect after infection has happened," he said.

The findings, reported in the Aug. 15 issue of the Journal of the American Medical Association, support a recommendation in June from the U.S. Centers for Disease Control and Prevention that vaccination focus on girls 11 and 12 years of age, most of whom would not have already become sexually active.

Gardasil, which targets four cancer-causing strains of the virus -- 6, 11, 16 and 18 -- should be added to the list of routine school vaccinations, experts say.

On the other hand, "for women who have initiated sexual activity, cervical cancer screening is probably a better preventive measure than vaccination," Hildesheim said.

His team's NCI-funded study, which focused on the Cervarix vaccine, was conducted in Costa Rica and included almost 2,200 women aged 18 to 25. Cervarix has not yet been approved by the U.S. Food and Drug Administration, but its maker, GlaxoSmithKline, has said it believes the shot will gain approval sometime in 2008.

According to Hildesheim, prior studies had already strongly suggested that both Gardasil and Cervarix would be ineffective against preexisting viral infections.

"However, we noticed shortly after the initial licensure of the [Gardasil] vaccine that there was tremendous confusion -- both in the clinical and the lay community, regarding the use of this vaccine among women who were already infected," he said.

Other specialists have noticed similar confusion among their peers.

"Each of us has received anecdotal reports of doctors thinking that you can benefit [an infected] woman by vaccinating her," said Dr. Howard Strickler, professor of epidemiology and population health at the Albert Einstein College of Medicine, in New York City. "There are misconceptions in the field," he said.

Even though HPV occurs in a wide variety of strains, Cervarix targets the two strains thought to cause 70 percent of cervical malignancies -- strains 16 and 18.

In the study, the researchers gave the vaccine to about 1,100 young women, all of whom had tested positive for genetic traces of HPV at the beginning of the study. Another group of infected young women received a hepatitis A shot as a "control."

By the end of 12 months, rates of HPV 16 and/or 18 "clearance" -- or absence -- from the body was 48.8 percent in the HPV vaccine group and 49.8 percent among the controls -- a statistical dead heat.

As the experience of the control group showed, the human immune system naturally clears many HPV infections over time. Giving women the vaccine appeared to provide no added benefit, the researchers said.

The investigators also observed no differences between the two groups in terms of the extent of HPV-linked disease, viral antibody load or the results of cervical cell tests conducted in the lab.

"Our results reinforce the notion that this vaccine is meant to protect against infections when they occur, but it isn't effective at helping clear the infection once it has established itself," Hildesheim said.

Still, given the array of HPV viral strains, wouldn't vaccination help protect against viral types a woman had perhaps not yet encountered? Theoretically, that's true, the experts said, and it is for exactly that reason that the CDC recommends the vaccine for women up to age 26.

However, there's currently no test that can determine a woman's HPV history.

"If you are sexually active, there is no way for you to know for sure whether or not you have been exposed to these particular [carcinogenic] strains," explained Hildesheim. They "are very common, and, typically, exposure happens fairly shortly after the initiation of sexual activity," he said. "That's why the best policy is to try and get the vaccine administered prior to sexual debut."

So, for both girls and women, vaccinated or not, the expert advice on avoiding cervical cancer remains the same: Get regular Pap smears.

"Remember, even [Gardasil] is just working against four strains," noted Dr. Stephanie Blank, a gynecologic oncologist and assistant professor of gynecologic oncology at New York University School of Medicine. "There are many more strains than that. So, the Pap smear recommendations have remained unchanged."

SOURCES: Allan Hildesheim, Ph.D., senior investigator, division of cancer epidemiology and genetics, U.S. National Cancer Institute, Bethesda, Md; Howard Strickler, M.D., M.P.H., professor, department of epidemiology and population health, Albert Einstein College of Medicine, New York City; Stephanie Blank, M.D., gynecologic oncologist and assistant professor, gynecologic oncology, New York University School of Medicine, New York City; Aug. 15, 2007, Journal of the American Medical Association