Jan 4, 2012 -- It’s back to the drawing board for researchers seeking to develop a vaccine that protects against genital herpes.

Earlier studies of the vaccine were promising, but the new report shows that an experimental genital herpes shot was only mildly effective against herpes type 1 (HSV-1) -- which commonly causes cold sores and sometimes genital herpes -- and not effective at all against herpes type 2 (HSV-2), the most common cause of genital herpes.

Vaccine manufacturer GlaxoSmithKline provided some funding for the new study.

Herpes virus is typically transmitted through sexual or skin-to-skin contact. It can spread even when the infected person has no symptoms. Infants born to women with herpes infections can contract herpes. HSV-1 is a growing cause of genital herpes in college students and young heterosexual women. This is why it is crucial that any vaccine protects against both HSV-1 and HSV-2.

More than 50 million adults in the U.S. have genital herpes, and up to 1.6 million new infections occur each year, according to the American Social Health Association.

The new study included more than 8,300 women ages 18 to 30 who were free of the herpes virus. Women received three doses of the new vaccine or a vaccine against hepatitis A, a virus that causes liver inflammation. Women were tracked for 20 months to see if the shot prevented herpes infection or disease.

The new vaccine was somewhat effective against HSV-1, but did not prevent infection with HSV-2, the new study showed. The findings appear in the New England Journal of Medicine.

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Still Looking for Answers

Researchers remain puzzled about why it didn’t work well. Previous studies with more positive results were conducted among couples in which one partner was infected and the other was not. This may play a role in why the results don’t match up.

“This is not the final vaccine,” says researcher Robert B. Belshe, MD. He is a professor of medicine, pediatrics, and molecular microbiology at Saint Louis University School of Medicine in Missouri.

The new study shows that it may be possible to develop a vaccine against HSV-1 and HSV-2, he says, but the next step is to determine why it was less effective at preventing HSV-2 than HSV-1.

“This information will help us a lot,” Belshe says. “There may be a very simple answer, or it may take a more complex vaccine with many components.”

John D. Kriesel, MD, says he wouldn’t bet on this happening anytime soon. He is an infectious disease specialist at the University of Utah School of Medicine in Salt Lake City. He helped recruit women to participate in the new study.

“It is very disappointing and perplexing that it didn’t work after the two other trials showed that it did work," he says. “The reason for the failure is not entirely apparent and probably never will be.”

“The landscape for an effective herpes vaccine is bleak to say the least,” Kriesel tells WebMD.

Bruce Hirsch, MD, also thinks we are far away from an effective vaccine against genital herpes. He is an infectious disease specialist at North Shore University Hospital in Manhasset, N.Y.

“This study looked at the vaccine in a large group of women that represent the kind we most want to protect, and it didn’t seem to be effective,” he says. 

Unless and until a vaccine is proven safe and effective, antiviral medications may offer some protection against transmission. “This is reassuring,” Hirsch says.