A research first: a vaccine that protects against genital herpes. A vaccine first: it only works in women, and only if they have never had cold sores.

Until now, no vaccine has ever been shown to work in one sex but not the other. Experts say this could present unexpected trouble for creating other vaccines for sexually transmitted diseases, STDs, like AIDS.

Why the vaccine works in women but not men is unclear, although the researchers said it probably has something to do with differences in sexual anatomy. They say it could be the vaccine boosts the immune system so it can attack the herpes virus while still in the vagina, but is unable to stop the virus after it gains access to the bloodstream through tiny tears in the penis.

But doctors say a vaccine offering even partial protection against a chronic disease is noteworthy. The only other sexually transmitted disease that can be stopped with a vaccine is hepatitis B.

Dr. Spotswood Spruance, one of the University of Utah researchers who tested the vaccine says he thinks the chances for approval are good. He predicted the herpes vaccine would be given to adolescent girls.

He said widespread use of the vaccine this way would probably reduce genital herpes for both sexes, since it would lower the chance of men coming in contact with infected females.

Genital herpes and cold sores result from closely related bugs. Herpes simplex virus type 1, or HSV-1, causes fever blisters on the mouth, while HSV-2 triggers sores on the genitals. Once acquired, both infections last a lifetime.

Two studies - interesting results

Spruance and colleagues reported the results of two major studies of the vaccine at a meeting in Toronto of the American Society for Microbiology.

Both were conducted on couples in which one partner had genital herpes but the other did not. The studies involved more than 2,700 people in the United States, Canada, Australia, Italy and New Zealand.

In one of the studies, the partners who were free of genital herpes had never been infected with either HSV-1 nor HSV-2, while the other included those who had HSV-1 but not HSV-2.

During 19 months of follow up, it turned out the vaccine did nothing to protect men or to protect women who already had HSV-1.

But it was about 75 per cent effective in warding off genital herpes sores in women who had never had either form of the virus.

The researchers said that being infected with HSV-1 probably helps protect people from getting genital herpes, and the vaccine does little to increase this natural barrier.

Gary Dubin who heads adolescent vaccine development for SmithKline Beecham in Belgium said the results were not exactly what they expected. He said the company is getting reaction from regulators and public health officials before deciding what to do next.

The vaccine is made from a protein taken from the outer surface of the herpes virus. It is combined with a bacterial toxin that acts as an immune system booster.