An experimental herpes vaccine protected young women against only one of the two types of the sexually transmitted virus, dashing hopes for widespread use of the treatment, researchers reported in the latest issue of the New England Journal of Medicine.

For reasons that aren’t clear, the vaccine protected against herpes simplex virus type 1, known as HSV1, but not type 2, known as HSV2, the study of more than 8,000 women aged 18 to 30 found.

“I think this is the end of the vaccine,” said coauthor Dr. Peter A. Leone, an infectious disease specialist at the University of North Carolina, Chapel Hill. “It would be difficult to imagine marketing a vaccine that would only work against HSV1.”

According to the Centers for Disease Control and Prevention, one in six Americans ages 14 to 49 is infected with HSV2. Nearly 60 percent of adults in the U.S. are infected with HSV1, federal health figures show.

Still, Leone and his fellow investigators called the vaccine against type 1 “a substantial step forward” in the quest for a future vaccine to protect against both types of herpes.

That's important because many people still think that the type 1 herpes virus causes only cold sores. “It used to be we’d think HSV1 above the waist, HSV2 below the waist,” Leone said.

In his study, though, HSV1 was a more common cause of genital disease in the women who didn’t get the herpes vaccine than HSV2. Scientists have assumed that people have to engage in oral sex to get genital HSV1 disease, Leone says, but his study didn’t find an association.

Women who weren’t infected with herpes at the beginning of the study were randomly assigned to receive either three shots of the herpes vaccine or three shots of the hepatitis A vaccine.

Two previous studies of the vaccine involved heterosexual couples in which either the man or woman was infected with herpes. Those found that the vaccine protected against both types of herpes in women, but neither of the types in men.

Perhaps having regular sex with an infected man primed the women’s immune systems to fight HSV1 and HSV2, or maybe they were naturally resistant, Leone and his coauthors theorized. Leone said it might not have worked for men because the skin covering the penis is different from the membranes lining the vagina and cervix.

So what’s next?

“We’re going to need a different approach,” Leone said. His trial used a vaccine containing an HSV protein designed to trigger an immune response against the virus. Maybe, he said, a vaccine that uses weakened live virus -- like the chickenpox vaccine -- would work better.

Meanwhile, Leone said, many Americans live in fear of contracting herpes. “The idea that you can transmit this and not know it terrifies people.”

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