A potential HIV vaccine manufactured by pharmaceutical company Merck did not protect against infection or reduce the virus level in early HIV infection, according to the results of a study published online in The Lancet.

Researchers from Merck Research Laboratories and the San Francisco Department of Public Health and their colleagues undertook a clinical trial, called the Step Study, to assess the efficacy of a vaccine against HIV infection. The Step Study took place at 34 sites in North America, the Caribbean, South America, and Australia and involved 3000 participants who were free of HIV infection. The participants were randomly assigned to receive 3 injections of either the vaccine or a placebo. The study was stopped early when it met prespecified criteria for futility. The researchers found that vaccine recipients actually had a higher rate of infection than participants who received placebo. Further analysis showed that the increased risk for HIV infection was only seen in uncircumcised men and men who already had antibodies to HIV.

The findings of the Step Study suggest that a new approach may be needed to develop an effective HIV vaccine. However, further exploration is being made into the reasons for vaccine failure and the increased risk for HIV infection observed among some men.