WEDNESDAY, Jan. 4 (HealthDay News) -- A potential vaccine for genital herpes has shown only limited effectiveness in thwarting one type of the sexually transmitted virus and no ability to stop a second type from spreading, a new study shows.

A group of American and Canadian researchers conducted a randomized trial on more than 8,300 women aged 18 to 30 who tested negative for both forms of the herpes simplex virus, known as HSV-1 and HSV-2. Half of the women were given the experimental vaccine while the other half were given the hepatitis A vaccine.

The experimental vaccine was 58 percent effective at preventing genital disease stemming from HSV-1, but completely ineffective against HSV-2.

"We were disappointed it did not meet the primary [goal], which was protection against all types of genital herpes," said study author Dr. Robert B. Belshe, a professor of medicine, pediatrics and molecular microbiology at Saint Louis University. "Herpes is a complex organism and has ways of escaping the immune system, so we have to figure out a way of overcoming those mechanisms."

The study is published Jan. 5 in the New England Journal of Medicine.

Genital herpes affects about 16 percent of Americans aged 14 to 49, according to the U.S. Centers for Disease Control and Prevention. Both types of the virus are released from their resulting sores but can also be transmitted between outbreaks, and the infection is potentially fatal in newborns who acquire it from their mothers during birth.

Fifty clinical sites in both the United States and Canada followed the women -- who reported their own sexual risk behaviors -- between 2003 and 2007. A heightened risk for HSV-1 infection was associated with six or more lifetime sexual partners and more than one partner in the previous 12 months, while those who were 23 or older were less likely to contract HSV-1 than those between 18 and 22.

Factors not associated with an increased likelihood of HSV-1 included race, condom use, oral sex, a history of any sexually transmitted infection or ever having a partner with herpes. Men weren't tested in this study, though prior research showed investigational herpes vaccines to be ineffective in men and HSV-1 positive women.

"I was very disappointed there wasn't more of a benefit," said 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. "I think we're asking that a vaccine provide better immunity than an actual infection provides. I'm concerned if we'll ever find a vaccine effective for HSV-2."

"I think what we tell patients is ... there are antiviral medications that can make a difference and can give people comfort, and the availability of treatment is encouraging," he added.

While other potential vaccines are in the pipeline, Belshe said, the one that works probably needs to be "more complex" than the one recently studied, which contained a single surface protein of the herpes virus. The chicken pox vaccine, which has been widely used in the past decade, is a good example of a herpes-related virus that has been brought under control, he said.

"Something like that is what we need to come up with for HSV," he said. "I think this is a very important study, and the result is an incredibly important step in figuring out what will work."

More information

The U.S. Centers for Disease Control and Prevention has more on genital herpes.