Vaccine clears major hurdle: injections offer new tool against cervical cancers.

An experimental vaccine against the virus that causes most cancers of the cervix has passed--with flying colors--a test typically needed for regulatory approval.

The vaccine targets four strains of the sexually transmitted human papillomavirus (HPV), which infects about 20 million men and women in the United States. Last week in San Francisco at a meeting of the Infectious Diseases Society of America, researchers conducting a large trial in women reported that the vaccine prevented all cervical cancers and precancerous lesions traceable to the two most dangerous strains of the virus.

The vaccine, called Gardasil by its maker Merck & Co., targets four viral strains. Two of them, HPV-16 and HPV-18, account for most cervical cancers. The other two strains cause genital warts, which are difficult to treat but don't become cancerous.

Over 6 months, researchers gave three injections to each of 10,559 sexually active women, ages 16 to 26, living in 33 countries. Half got the vaccine; half received a placebo. Roughly 18 months after the last injection, none of the vaccinated women had developed cervical cancer or precancerous lesions stemming from HPV-16 or HPV-18, whereas 21 of the women getting the placebo injections had, the international team reported. The scientists didn't release data on genital warts.

A pap smear can detect precancerous growths on the cervix arising from HPV. Although such growths can be removed surgically, a better approach would be to avoid infection in the first place, says vaccine researcher William Schaffner of Vanderbilt University School of Medicine in Nashville. "Here, we have an opportunity to try to prevent cancer," says Schaffner, who wasn't involved in the study.

While the study didn't indicate how long the vaccine's protection will last, researchers in Scandinavian countries involved in the study plan to track the participants for years using those countries' national cancer registries.

"That will be our best data for [determining] long-term efficacy" of the vaccine, says study coauthor Kevin A. Ault of Emory University School of Medicine in Atlanta.

Merck plans to submit the vaccine to the Food and Drug Administration for licensing by the end of this year. If it's approved, there will be a "catch-up time" when women of any age who are sexually active could get vaccinated, Ault says. Afterward, vaccine distribution will target girls before they become sexually active, he says.

The vaccine could prevent three-fourths of all cervical cancers worldwide, Schaffner estimates.