An experimental vaginal gel containing a drug used to treat the AIDS virus could prevent half of cases of genital herpes, according to a study done in South Africa.

Among women who used tenofovir gel, the annual rate of infection with the genital herpes virus, known as herpes simplex virus type 2 or HSV-2, was 10.2 percent versus a rate of 21 percent for women who used a placebo gel.

If the gel were to be approved and manufactured, and "if a woman was concerned about acquiring herpes simplex virus type 2, this would be the best protection available," chief author Dr. Salim Abdool Karim of the Centre for the AIDS Program of Research in South Africa told Reuters Health.

"The problem is, right now it's not available. And taking tenofovir tablets instead of the gel doesn't have the same benefit," he said. "You see some protection with tablets, but the levels of protection are much lower."

HSV-2 virus infects about one in five sexually active adults worldwide, or about 417 million people aged 15 to 49 years old, the study team writes in the New England Journal of Medicine. The virus is the most common cause of genital ulcers, and in South Africa, the estimated infection rate is 50 percent to 60 percent. Throughout Sub-Saharan Africa the rate is thought to be as high as 80 percent in women and 50 percent in men.

Condoms provide the best protection, but even that is limited because the virus can be shed and acquired by other parts of the groin not shielded by a condom, Abdool Karim said.

Oral tenofovir is used to prevent and treat HIV. The test of its ability to protect against genital herpes was grafted onto an existing study, which ultimately demonstrated that the gel reduced the odds of getting HIV by 39 percent.

Among the 422 women enrolled in the herpes study, half were given applicators with the drug-infused gel and the rest were given a placebo gel. They were told to insert the gel within 12 hours before they expected to have sex and as soon as possible within 12 hours after sex. The women were tracked for an average of 18 months.

The study results indicate that topical tenofovir can reduce HSV-2 acquisition by almost half, said Dr. Connie Celum, director of the International Clinical Research Center at the University of Washington in Seattle, by email.

"This is significant, given the lack of other interventions to reduce HSV-2 acquisition other than condoms," said Celum, who was not connected with the study.

The drug worked best among women who used the gel religiously. When drug levels in the vagina were high, the annual rate of herpes-2 infection was less than 6 percent compared to almost 16 percent when there was no detectable tenofovir in the vaginal fluid.

The study did not detail side effects, although an earlier report available online says the drug can increase the risk of mild diarrhea.

"Because it's in the gel formulation, it leads to very high levels in the genital tract and very low levels in the blood, so very little is absorbed and you get very limited side effects," Abdool Karim said. "We did see a slight increase in mild diarrhea and it wasn't statistically significant."

There was also concern that the gel might show adverse effects in the kidneys and bones, he said, but "with the gel formation, we didn't see either of those problems."

A key question is whether women will actually use it when the gel gets to market.

"We've had a mixed bag of results in trying to get women to use tenofovir gel for HIV protection," said Abdool Karim. "The main concern has been many of these young individuals who are at highest risk for herpes and HIV don't actually consider themselves at risk. They don't appreciate their risk. It's the same for alcohol consumption, dangerous driving or smoking or any of these things."