Merck has been testing a candidate HIV vaccine in two large phase II clinical trials; the STEP study in North and South America, the Caribbean and Australia, and a parallel study called Phambili in South Africa.
STEP was a multi-center, randomized, double-blind, placebo-controlled phase II test-of-concept clinical trial. Started in December 2004, enrollment in the study was completed by March 2007 and more than 2,500 participants had received all three doses of vaccine or placebo. STEP was evaluating two primary efficacy endpoints:
- Whether the vaccine prevented HIV infection in subjects who were HIV negative at the beginning of the study.
- Whether the vaccine lowered the amount of virus in subjects who became HIV infected during the course of the study.
However, last month the drug trial was halted because the vaccine appeared to be increasing the risk of HIV infection among study participants. The South African study Phambili, which began in 2007 using the same vaccine has been halted .