An experimental vaccine to prevent the most common forms of cervical cancer proved 100 percent effective in a two-year test on more than 10,000 girls and women, drug maker Merck & Co. says.

Merck is hoping to win Food and Drug Administration approval for the vaccine, Gardasil, and put it on the market as soon as late 2006. It would be the first vaccine to prevent cervical cancer, a disease caused almost exclusively by a highly common sexually transmitted virus called the human papilloma virus, or HPV.

Doctors expect the vaccine to be routinely offered to girls – and boys, too, because they can spread the virus to their partners – before they become sexually active, though the practice is certain to run into opposition from conservatives and religious groups.

“I see this as a phenomenal breakthrough,” said Dr. Gloria Bachmann, director of the Women’s Health Institute at Robert Wood Johnson Medical School in New Brunswick, N.J.

Worldwide, cervical cancer is one of the most common cancers among women. It kills nearly 300,000 a year, including about 3,700 in the United States. About 20 million Americans have some form of HPV, which in addition to cervical cancer can cause painful genital warts.

The genetically engineered vaccine prevents cervical cancer by blocking infection from the two strains of HPV that cause 70 percent of all cases of the disease.

The study included 10,559 sexually active women ages 16 to 26 in the United States and 12 other countries who were not infected with either of the two virus strains, called HPV 16 and 18. Half got three vaccine doses over six months; half got dummy shots.

Among those still virus-free after the six months, none of those who received the vaccine developed either cervical cancer or precancerous lesions during two years of follow-up, compared with 21 of those who got dummy shots.

“To have 100 percent efficacy is something that you have very rarely,” said Dr. Eliav Barr, Merck’s head of clinical development for the vaccine. “We’re breaking out the champagne.”

The study, which was funded by Merck, will be presented today at a meeting of the Infectious Diseases Society of America.

A second analysis showed that after just one dose, the vaccine was 97 percent effective. Barr said the 97 percent rate was more “real world,” given that patients sometimes miss or delay follow-up shots.

Barr also noted that a small number of women in the study developed dangerous precancerous lesions caused by HPV types other than 16 and 18.

The immune system clears most HPV infections in a year or two, but several types of HPV can persist, cause cervical cancer or trigger other cancers in the genital area. There is no cure for HPV, but the cancers can be treated and an improved Pap test is catching more cases of cervical cancer before it spreads.

Bachmann said that to fight the disease, students would have to be vaccinated in high school, middle school, even elementary school, before they become sexually active.

Merck, hammered by slumping profits and facing roughly 5,000 lawsuits over its withdrawn painkiller Vioxx, is seeking to beat rival drug maker GlaxoSmithKline to market with the first cervical cancer vaccine.