Efforts to develop the world's first vaccine to prevent cervical cancer (search) took a key step forward Monday with test results suggesting that it can provide long-lasting protection.

Four years after getting the vaccine, 94 percent of women were protected from infection with the virus that causes most cervical cancers and none had developed worrisome precancerous conditions, a study showed.

"We're thrilled about these results. The immune responses seem to be really long-lasting," said Dr. Eliav Barr, who leads development of the vaccine for Merck & Co.

The company plans to seek U.S. Food and Drug Administration approval next year for an expanded version of the vaccine that also could be used to prevent genital warts in both women and men.

"They showed clear effectiveness," said Dr. Scott Hammer, a Columbia University infectious disease expert who reviewed the work but has no ties to Merck or the study. "This is a very important issue for women's health around the world."

If the vaccine makes it to market, it would be the second developed to prevent cancer. The hepatitis B vaccine (search) has dramatically reduced the number of infections that progress to liver cancer.

Cervical cancer strikes nearly half a million women worldwide each year and kills about half. In the United States, about 15,000 women get it and about 5,000 die.

Virtually all cases are caused by infection with human papilloma virus (search), or HPV, which is spread through sex. One strain, HPV-16, accounts for about half of all cervical cancers.

A previous study showed that HPV-16 infections were completely prevented in 768 women who had received the Merck vaccine 18 months earlier. None developed precancerous conditions either.

The new study followed 755 of these women for four years after vaccination. HPV-16 infections had taken hold in seven; none developed precancers. In a comparison group of 750 women who received dummy shots, infections took hold in 111 and precancers formed in 12.

Even though protection had waned for a small number of women in the study, the vaccine's effectiveness was still very high, said Dr. Douglas Lowy, a National Cancer Institute scientist who invented the vaccine. The government gave rights to develop it to two companies — Merck and GlaxoSmithKline — and Merck's work is a little farther along, Lowy said.

"Revaccination might be advisable at some point," but it will take more study to know whether that is necessary or would improve effectiveness, he said.

Women in the study were ages 16 to 23 when they received the vaccine, given in three doses over six months.

"Most people think it would be recommended for young adolescents. The idea would be you would immunize people relatively soon before they become sexually active," because the germ is spread through sex, Lowy said.

Meanwhile, Merck is in the final stages of testing an expanded vaccine. Besides HPV-16, it contains strain 18, which causes another 10 percent to 20 percent of cervical cancers, as well as strains that cause genital warts in men and women, and penile and anal cancers in men.

About 25,000 women and men have been enrolled in that study in 34 countries, and results are expected next year, Barr said.

Giving the vaccine to men would not only prevent disease in them but also would prevent infections in their partners, said Steven Projan, a drug development expert with Wyeth who helped review research for the microbiology meeting. The vaccine also might prevent women already infected with HPV from developing cancer, he said.