An experimental vaccine has proved highly effective at preventing cervical cancer in a two-year study involving more than 12,000 women, researchers reported yesterday.

The vaccine works by making people immune to two types of a sexually transmitted virus that cause most cases of the disease. It is the first successful vaccine ever developed specifically to prevent cancer.

The vaccine, Gardasil, is made by Merck & Company, which plans to apply for approval to the Food and Drug Administration before the end of this year and, if the vaccine is approved, to market it in 2006.

If widely used, the vaccine could save many lives. Worldwide, there are about 500,000 new cases of cervical cancer a year, and 290,000 deaths. Most of the cases and most of the deaths occur in poorer countries where women do not have regular Pap tests, which can detect cancers or precancerous cells early enough for them to be cured. In the United States, where Pap tests are common, 10,400 new cases are expected in 2005, and 3,700 deaths.

"The potential, particularly in the undeveloped world, particularly if they can overcome the logistics and get the vaccine to those women, could be enormous," said Dr. Deborah Saslow, director of breast and gynecological cancer at the American Cancer Society. The vaccine could prevent at least 70 percent of the deaths from cervical cancer, Dr. Saslow added.

But Dr. Allan Hildesheim, an epidemiologist at the National Cancer Institute, cautioned that even if women are vaccinated, they will still have to be screened regularly for cervical cancer because the vaccine does not prevent all cases of the disease.

"This is not a panacea," Dr. Hildesheim said.

The vaccination will require three shots over six months. Merck has not said what it will cost.

The ideal time to vaccinate girls is before they become sexually active and risk being exposed to one of the cancer-causing viruses, said Dr. Eliav Barr, a research director at Merck. Once cancer develops, it is too late for the vaccine to help. The median age at which girls first have sex in the United States is 15.

It is not known yet how long the protection from the vaccine will last, or whether booster shots will be necessary, Dr. Barr said.

The vaccine works against viruses that belong to a group called human papillomaviruses, or HPV. Nearly every case of cervical cancer is caused by HPV. The viruses are sexually transmitted, extremely common and almost impossible to avoid. At least half the adults in the United States have been infected.

More than 30 types of HPV infect the human genital area. Only some types cause cancer; others cause genital warts. A type known as HPV-16 causes 50 percent of cervical cancers, and HPV-18 causes 20 percent. Other types cause the rest. But even the cancer-causing types are harmless in most people because their immune systems fight them off.