The lifelong infection affects 1 in 5 adults, and many infected people don't have symptoms and don't know they are infected.
In the study heterosexual, monogamous couples were instructed to use condoms during sex. Previous research has shown that even when told to use condoms, many monogamous couples don't follow these instructions.
Once-daily Valtrex cut the chance of the partner getting herpes with no symptoms by 50%. The risk of developing genital herpes with symptoms in the partner dropped by 77%.
"This is the first time that a drug has been shown effective against the transmission of virally transmitted infections," lead researcher Larry Corey, MD, tells WebMD.
Corey is professor of laboratory medicine and medicine at the University of Washington and head of infectious diseases at the Fred Hutchinson Cancer Research Center in Seattle. He presented his findings today at a meeting of infection disease specialists in San Diego.
"This is a breakthrough concept," he says. "It could offer a means of managing other sexually transmitted diseases.
"Condoms have been only partially effective in preventing transmission," says Corey. "It could be that most monogamous couples use condoms for contraception rather than to prevent transmission of infection. So condoms aren't necessarily always put on in the really early stages of the sexual event. That may be a factor for herpes. In that case, other approaches of prevention are necessary."
Surveys over the years have shown that one of the major concerns most people have is transmitting genital herpes to a sex partner. "We now have a drug that for the first time allows a person to use medication as one form of decreasing transmission," he says.
Because the couples were asked to use condoms every time the had sex, some of the reduction in transmission may be due to condom use, not Valtrex alone.
Corey's study was conducted in 126 sites on four continents. Researchers identified more than 1,400 couples in which one had genital herpes and one did not. In half the couples, the person with herpes got Valtrex; in the other half, the infected person got a placebo. During the eight-month study period, the other partner was then monitored to find out if they had become infected.
The findings are "pretty convincing," Clyde Crumpacker, MD, an infectious disease specialist with Boston's Beth Israel Deaconess Medical Center and professor of medicine at Harvard Medical School, tells WebMD.
"I think it's the first time this has been tested in a study as rigorous as this," says Crumpacker, who agreed to review the study for WebMD.
An estimated nine out of 10 people are unaware they have genital herpes and may only experience a mild initial outbreak without recognizing recurring symptoms of the disease.
"It's called asymptomatic shedding," says Corey. "People have no sign of herpes but will shed the virus when they have sex. It's possible that the virus is transmitted when people are not feeling infected at all."
The only way to know if you are infected is to get an antibody test -- a simple blood test that can be obtained at a clinic or doctor's office, he says.
"Genital herpes is the most common cause of ulcers in the genital area," Crumpacker says. "First, you develop red bumps in the genital area. For men, it's the shaft of the penis; for women, it's the inner labia or outer vagina. The bump is painful and itchy, and will break down into an ulcer. That's when the virus is most transmissible. From 10-12 days later, it will crust over and heal up."
There's no permanent way to get rid of the virus, but people who have frequent recurrences can take Valtrex daily to prevent recurrences. Corey's study shows "Valtrex provides another safeguard," he says. "If they take the pill all the time, they can reduce the chances of the virus shedding and being transmitted to someone else."