By Maggie Fox, Health and Science Editor

WASHINGTON, Feb 4 (Reuters) - People who took a drug to reduce outbreaks of genital herpes were not any less likely to become infected with the AIDS virus, an international team of researchers reported on Monday.

The findings raised questions about whether the drug, called acyclovir, worked well enough to stop blistering or whether herpes raises the risk of infection in ways not fully understood.

Connie Celum of the Fred Hutchinson Cancer Research Center in Seattle and colleagues tested more than 3,000 volunteers with genital herpes, also known as herpes simplex 2. Half got acyclovir and half took placebo pills.

This herpes virus, which infects an estimated 45 million people in the United States alone, has been shown to raise the risk of HIV infection by as much as 69 percent.

It causes painful, suppurating blisters and one obvious theory for why people with herpes are also more likely to get HIV was that the open sores give the AIDS virus a doorway into the body.

"It makes sense to see if treating herpes might reduce the risk of acquiring HIV," said Dr. Rowena Johnston, vice president of research for amfAR, The Foundation for AIDS Research, a nonprofit group.

"The question was if we treated them for genital herpes did it decrease the risk of acquiring HIV. The answer was no. It is is a disappointment," Johnston said in a telephone interview.

Celum told a meeting of AIDS researchers in Boston that there was no difference in the rate of new AIDS infections among their volunteers in Peru, South Africa, Zimbabwe, Zambia and the United States.

Johnston, who attended the presentation, said it was possible that the drug either did not work well, or people did not take the daily two pills regularly enough to keep the herpes virus under control.

There is no cure for herpes, but drugs such as acyclovir and famcyclovir can suppress it and prevent outbreaks.

"They had a 37 percent reduction in the incidence of genital sores in the acyclovir arm of the study," Johnston said.

"We know the acyclovir was somewhat effective. We know that people were taking their pills to some extent."

But, she added, newer herpes drugs are more effective so perhaps acyclovir was not the best drug to test.

But she has another theory. The AIDS virus attacks immune system cells, and because herpes infections are permanent, they may keep those immune cells actively circulating in the body.

"If you have an ongoing immune reaction in your genital tract it is possible you have an ongoing risk of acquiring HIV," she said.

The AIDS virus infects an estimated 33 million people globally. It has killed 25 million and there is no cure and no vaccine. Condoms can protect people from the sexually transmitted disease and a cocktail of drugs can keep it under control.

(Editing by Will Dunham; Editing by Cynthia Osterman)

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