Vaccine could prevent most cervical cancers. (Virus Stopper).

Throughout much of the 20th century, scientists suspected that sexually transmitted infections cause cancer of the cervix. But the culprit remained hidden until 2 decades ago, when scientists isolated human papillomavirus (HPV) DNA from cervical tumors.

That discovery is now paying dividends. In the Nov. 21 New England Journal of Medicine, a team of U.S. scientists reports that a vaccine fashioned from an HPV protein protects women from long-term viral infections that can lead to cervical cancer.

"This is an amazing accomplishment," says medical geneticist Robert D. Burk of the Albert Einstein College of Medicine in New York. "It represents what modern molecular genetics can do."

Scientists at Merck Research Laboratories in West Point, Pa., made the vaccine by mass-producing one of two proteins that form a shell around a virus called HPV-16. This virus type is found in roughly half of all cervical tumors. Without viral DNA to accompany it, the protein--called L1--can't cause disease. A safety test on a handful of volunteers confirmed that and indicated that the L1 vaccine primes a person's immune system to make antibodies against HPV-16, says study coauthor Laura A. Koutsky, an epidemiologist at the University of Washington in Seattle.

To test the vaccine's effectiveness in a large group, Koutsky and her colleagues tracked 1,533 women. They were between the ages of 16 and 25, had never had abnormal cervical cell growth show up in a Pap smear, and had blood that tested negative for HPV-16 antibodies. Half received three injections of the vaccine over 6 months, while the others got shots of an inert substance.

Since their injections, the women have undergone periodic health tests including Pap smears. After an average of 17 months, none of the women receiving the vaccine tested positive for HPV-16, but 41 of the women getting the placebo did. During this follow-up period, nine women showed abnormal cell growth on their cervices. They were all in the group that received placebo injections. Such cell growth is sometimes precancerous.

Women receiving the vaccine had 59 times as much antibody against HPV-16 in their blood as did women who had HPV-16 infections at the initial screening for study participation.

Responsible for roughly 250,000 worldwide deaths every year, cervical cancer kills more women than any other cancer in developing countries where Pap smears are infrequent. In contrast, ifs only the 12th-most-lethal malignancy in U.S. women because most cases are detected and treated.

There are more than 60 types of HPV, at least 20 of which are linked to cervical cancer. After HPV-16, the most common type found in tumors is HPV-18. Some other HPV types, notably HPV-6 and -11, cause genital warts but haven't been linked to cancer. Merck is developing a vaccine that would combine proteins from these four types. The company plans to test the vaccine in women and men because both can get genital warts and spread the cancer-causing HPV types.