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(CBS) An experimental herpes vaccine failed to protect young women from the disease in a new study, dashing hopes that a vaccine may soon be available to help prevent the sexually transmitted disease.

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In a study of 8,300 women ages 18 to 30, the vaccine, developed by GlaxoSmithKline, partially protected against HSV-1, the strain of herpes which typically causes cold sores but can also cause genital herpes. The vaccine, however, did not protect against HSV-2, the strain that most commonly causes genital herpes.

“There is some very good news in our findings. We were partially successful against half of the equation – protecting women from genital disease caused by HSV-1,” study author Dr. Robert Belshe, director of the Saint Louis University Center for Vaccine Development, said in a written statement.

None of the women had any strain of herpes when the study began, and half of the women were given the experimental vaccine while the other half were given a hepatitis a vaccine. Doctors monitored the women for twenty months, and throughout the study, some women got herpes.  The experimental vaccine was 58 percent effective at preventing genital herpes from HSV-1, but did not show protection against HSV-2. The study was published in the Jan. 5 issue of the New England Journal of Medicine.

Belshe said in the statement he was surprised the vaccine protected against one type of vaccine and not the other, but thinks that finding could lead to more research to better understand the disease.

“This is not the final vaccine,” Belshe told WebMD. “This information will help us a lot.”

Some experts aren’t as optimistic.

“I was very disappointed there wasn’t more of a benefit,” Dr. Bruce Hirsch, an attending physician in infectious diseases at North Shore University Hospital in Manhasset, N.Y., who wasn’t involved in the study, told HealthDay. “I’m concerned if we’ll ever find a vaccine effective for HSV-2.”

“Enough vaccines have failed that companies are seeing herpes vaccines as a risky business,” added Dr. David Knipe, a virologist at Harvard Medical School in Boston. Knipe told Nature News he thinks the public sector might need to fund herpes vaccine research to further advance it.

More than 50 million U.S. adults have herpes, according to WebMD, with up to 1.6 new infections occurring each year. Abstinence is the only way to prevent the STD, and latex condoms can reduce the risk of transmission, but the disease still can spread with condom use. The CDC says persons with herpes should abstain from sexual activity with uninfected partners when lesions or other symptoms are present. But even without symptoms, herpes can still spread to a partner.

The CDC has more on genital herpes.