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

Merck is hoping to win Food and Drug Administration (FDA) 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. But the practice is running into opposition from conservatives and religious groups.

To work, the vaccine ideally should be administered before people become sexually active, so parents may be called on to decide if they should vaccinate children as young as 9 or 10 against a sexually transmitted disease.

“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. She was not involved in the research.

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. HPV does not always cause Cervical cancer.

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 HPV 18.

Half received a three-dose regimen of the vaccine over a six-month period, while half received a placebo. The researchers detected no significant side effects.

Because Cervical cancer takes years to develop, the researchers looked at abnormalities in the cervix called precancerous lesions, which almost always progress to cancers if left untreated.

Over the two years of the study, there were no precancerous lesions associated with HPV 16 or 18 among the women who received all three doses of the vaccine. There were 21 precancerous lesions among those who received placebos.

“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.”

A second analysis showed that after 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 University of Washington was one of 90 centers worldwide to test Gardisil. Dr. Laura Koutsky, a UW professor of epidemiology, was one of the lead investigators. She was one of three professors who formed an HPV Prevention Program at the university in 1996 and has been researching HPV infections and related cancers for more than a decade.

The study, which was paid for by Merck, will be presented today at a meeting of the Infectious Diseases Society of America in San Francisco. Koutsky is expected to present some of the findings.

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. Indeed, the death rate from Cervical cancer in the United States is low because the disease is usually detected by Pap smears before it advances.

Bachmann, of the Women’s Health Institute, said that to fight the disease, youngsters would have to be vaccinated before they become sexually active, in high school, middle school, even elementary school.

Conservative groups including the Family Research Council have raised concerns that giving a sex-related vaccine to young people might encourage them to have sex.

Joanne Swift, an obstetrician-gynecologist in New Jersey, said yesterday that she had qualms about having her own daughters, ages 9 to 13, vaccinated.

“As a mother of children in Catholic school, they are very much lectured at home and at school about chastity and the benefits of that,” she said.

“I guess I would probably sell it to them by saying if you could preserve your own chastity, you can’t depend on the person with whom you’ve decided to spend the rest of your life.” In other words, there’s no guarantee the future husband didn’t contract the virus before marriage.

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, Cervarix.

GlaxoSmithKline is still enrolling patients in final-stage tests to determine whether its vaccine prevents cancer, and does not expect to have initial data until late next year. Spokeswoman Danielle Halstrom said earlier research showed it has a 100 percent success rate in blocking two virus strains, HPV 16 and 18.

The Merck vaccine was also found to reduce infection from two other HPV strains that cause 90 percent of genital-warts cases.

Questions about the drug remain. A critical one is whether the vaccine will retain its effectiveness for many years. It can’t be shown as yet that girls vaccinated at 9 would remain protected into adulthood; it is also not known if the drugs could be administered in infancy for a lifetime of protection.

Merck is continuing research on Gardasil and will soon report on four years of follow-up on the women in this latest study. In addition, Merck has been studying Gardasil’s effectiveness in boys 9 to 15.

Seattle Times reporter Cara Solomon contributed to this report.

Material from Knight Ridder Newspapers and The Los Angeles Times is included in this report.