HIV vaccine failure prompts Merck to halt trial

An HIV vaccine being developed by Merck has apparently failed, causing the company to halt a large and once-promising clinical trial last week.

Merck's STEP vaccine used a mixture of components from three weakened adenoviruses to carry three synthetically produced HIV genes. The hope was that each gene would stimulate an immune response against the virus, as earlier trials had suggested.

The latest trial began in 2004 and enrolled 3,000 people considered to be at high risk of infection. But a group of 741 volunteers who received the vaccine saw 24 HIV infections, compared with the control group of 762 people who saw 21 infections. Furthermore, the vaccine did not reduce the amount of HIV in the bloodstream of those infected.