GlaxoSmithKline’s (NYSE:GSK) experimental vaccine intended to prevent genital herpes in women failed in a clinical trial, according to the National Institutes of Health.

Of the 8,323 herpes-free women tested in the phase 3 Herpevac Trial, estimated effectiveness was 20%, though the NIH said all estimates have statistical uncertainty, and the effect was not “substantially different” from zero.

“The investigational vaccine was ineffective in protecting against genital herpes disease,” NIH said in a statement, though it is unknown at this time why the vaccine failed.

In two earlier studies of the drug involving men and women who did not have genital herpes but whose sexual partner were known to be infected, the vaccine prevented the disease in more than 70% of the female volunteers, though it had no clear effect in men.

The prior studies led to the larger Herpevac study that proved ineffective.