Genital herpes is an STD (sexually transmitted disease) caused mostly by HSV-2. It is a contagious viral infection affecting primarily the genitals of men and women. Genital herpes is characterized by recurrent clusters of vesicles and lesions at the genital areas or below the waist.
Researchers tracked more than 3,000 men and women infected with herpes in Africa, Peru and the United States. Roughly half were treated with aciclovir, an antiviral that stops herpes ulcers.
After a year and a half, the scientists found that 75 people out of the 1,581 who had been receiving aciclovir were later infected with HIV. Of the 1,591 people who received placebo pills, 64 contracted HIV.
In the trial, researchers chose participants who had herpes and who faced similar risks of getting AIDS. The study also questioned the participants during monthly visits about risky sexual behaviour with their recent partners.
The findings were published Friday in the medical journal the Lancet.
"It's a significant, disappointing finding," Francis Ndowa, co-ordinator of the sexually transmitted infections control team at the World Health Organization, said in an interview.
A small study in Tanzania last year also suggested that treating herpes didn't help reducing HIV susceptibility.
But that doesn't necessarily mean that the theory of treating herpes to avoid HIV is incorrect, experts said.
"It's probably likely that we need considerably more potent interventions than we have," Connie Celum, a professor of global health and medicine at the University of Washington who led the Lancet study, told the Associated Press.
Experts said there was a complex relationship between the two viruses that is still not entirely understood.
When herpes ulcers erupt, that draws white blood cells to the skin to fight the virus. Unfortunately, those white blood cells also have receptors for HIV.
Ndowa said it was possible that even without the telltale ulcers, herpes might have lingered while patients were taking the aciclovir, allowing HIV to gain a foothold.
In an accompanying commentary in the Lancet, Ronald H. Gray and Maria J. Wawer of Johns Hopkins University said it was questionable whether controlling sexually transmitted infections could work to prevent HIV.
"It is time to reassess the hypothesis and to adjust prevention policy accordingly," they wrote.
But Ndowa and Celum said the strategy of fighting herpes to prevent AIDS might work, if a different dosage or more powerful drug was tried.
"We don't exactly know why this didn't work, but this approach still has potential," Ndowa said. "Maybe it was just too much to expect from a tablet taken twice a day, that it could be effective against HIV."