GlaxoSmithKline has decided not to pursue further worldwide development of Simplirix, an experimental vaccine intended to prevent genital herpes disease in women. 

The decision was made after studying the results of a Phase III trial evaluating efficacy of Simplirix, which was conducted collaboratively with the National Institute of Allergy and Infectious Diseases, which is part of the US National Institutes of Health.

The trial began in 2002 and 8,323 women aged 18-30 were vaccinated at 50 sites in the USA and Canada. The data showed that the vaccine had an acceptable safety profile, but the primary trial endpoint, ie prevention of genital herpes, was not met.

Specifically, NIAID noted that the estimate of vaccine effectiveness was 20%, "but all estimates have statistical uncertainty, and this effect was not substantially different from zero". GSK and NIH said they will continue to evaluate data from the trial and plan to present details of the analysis in the near future "in an appropriate scientific forum".