GlaxoSmithKline Plc., which developed the vaccine and paid for the study, said the results were so promising it was launching a new final-stage study of the vaccine that will involve 7,550 women aged 18 to 30 around the United States.

If the new tests bear out the existing study results, the vaccine could be available in about five years, said Lawrence Stanberry of the University of Texas Medical Branch in Galveston, who led the study, published in Thursday's New England Journal of Medicine (news - web sites).

The vaccine prevented infection in 74 percent of women exposed for the first time to the genital herpes virus, known as herpes simplex virus type 2.

But the vaccine didn't work at all in men, researchers reported, adding they were not sure why.

The sexually transmitted disease can cause painful itching and sores that in some cases reappear for years. But most of the 45 million infected Americans have no symptoms, which helps spread the disease between sexual partners, who are often unaware they are carrying the virus.

The illness is particularly serious for infants, who get the virus from their mothers during birth. Half of the babies infected in this way die unless treated aggressively with an antiviral drug, Stanberry said.

Between 2,500 and 3,000 cases of neonatal herpes occur in the United States each year. "This vaccine is a leap forward in preventing neonatal infection, so it has some real added benefits," he said.

Herpes is also a risk factor in the spread of HIV (news - web sites)/AIDS (news - web sites) in adults.

The Stanberry team's test included 978 women and 1,736 men whose partners had genital herpes, all of whom got either three vaccine shots or three placebo shots within a six-month period. They were followed for a total of 19 months.

The researchers discovered the vaccine didn't work well in women who had been infected by a related virus, the herpes simplex virus type 1, which is responsible for cold sores or fever blisters.

Only when women had never been exposed to type 1 or type 2 herpes did the effectiveness of the vaccine reach 74 percent.

Stanberry said even with those limitations and the lack of effectiveness in men, the vaccine could be a powerful weapon in the fight against herpes.

"If you did universal vaccination of 11 and 12-year-old women, you would eventually see an impact on the spread of herpes in both men and women," he said.