A proposed vaccine against sexually transmitted herpes virus failed in a large clinical trial as it only partially protected women from one of two virus types, researchers said.

Previous studies of the vaccine were encouraging, but the new report showed that an experimental genital herpes vaccine was just somewhat effective against one strain of the STD, herpes type 1 (HSV-1).

The vaccine was designed to protect unaffected women against both HSV-1 and HSV-2. However, after testing 8,323 women, the vaccine was found to be only partially effective at preventing type 1 herpes, and so because of the shots limited protection it will not be tested in clinics.

Funding for the study came from the National Institutes of Health and GlaxoSmithKline.

Herpes virus is generally transmitted sexually or with skin to skin contact, and can spread even when the infected person has no symptoms. HSV-1 virus is growing among college student and young heterosexual women.

According to the American Social Health Association, more than 50 million adults in the U.S. have genital herpes, and up to 1.6 million new infections occur each year.

The findings were published in the New England Journal of Medicine