The vaccine protects against the human wart virus known as human papilloma virus 16, according to press reports. HPV 16, which causes genital warts, is linked to most occurrences of cervical cancer.

HPV Connection

"Scientists believe that some of these viruses may cause the growth of abnormal cells in the cervix and may play a role in cancer development," according to the National Cancer Institute Web site.

"They have found that women who have HPV or whose partners have HPV have a higher-than-average risk of developing cervical cancer," says NCI.

The results of the trial were particularly encouraging: Of the 755 women in the study, 94% did not contract HPV. Among those who did contract the virus, there were no instances of precancerous conditions.

In a control group of 750 women who received a placebo, 111 became infected by HPV, and 12 developed precancerous cells.

Exact Role of Viruses Unclear

"However, most women who are infected with HPV do not develop cervical cancer, and the virus is not present in all women who have this disease," says NCI. "For these reasons, scientists believe that other factors act together with HPVs. For example, the genital herpes virus also may play a role. Further research is needed to learn the exact role of these viruses and how they act together with other factors in the development of cervical cancer."

The vaccine used in the successful trials is not broad enough for widespread use, and Merck will be working on a more advanced vaccine, Dr. Eliav Barr of Merck told Reuters.