Promising test results on an experimental vaccine against the common, sexually transmitted human papillomavirus (HPV) suggest it may limit cases of cervical cancer. In an ongoing clinical trial, Gardasil, manufactured by Merck & Co., has prevented two common types of the virus. HPV is responsible for virtually all cases of cervical cancer as well as genital warts. Merck plans to apply for Food and Drug Administration approval by the end of the year; the company says the vaccine might be available by late 2006.

Currently, the best protection against cervical cancer is a routine Pap smear – but that requires women to schedule regular visits to a doctor. The effect of the 1941 introduction of the Pap smear can be easily seen: Between 1955 and 1992, the number of cervical cancer deaths in the United States dropped by 74 percent, reports the American Cancer Society. There will be about 10,370 new diagnoses of cervical cancer in the United States this year, estimates the society; about 3,710 will die from the disease this year.

Results from the study, involving more than 12,000 women ages 16-26 in 13 countries, show Gardasil prevented all noninvasive cervical cancers and high-grade cervical precancers linked to HPV types 16 and 18. Together these account for about 70 percent of cervical cancers, according to Merck. Researchers found that the 6,082 women given three doses of Gardasil over six months remained free of the two HPV strains after an average of 17 months; no high-grade cervical precancers or noninvasive cancers caused by these strains were seen in this group. In the placebo group of 6,075 women, 21 had such changes.

Maximum vaccine effectiveness “will depend on virtually 100 percent vaccination of men and women at a very early age” – probably no later than age 10, said Kenneth Noller, chairman of the department of obstetrics and gynecology at Tufts University School of Medicine, who said he has no ties to vaccine manufacturers or pharmaceutical companies. It is not known if the vaccine would benefit women who have already had HPV. At least 80 percent of women will have had HPV by age 50, reports the Centers for Disease Control and Prevention.