The clinical trial of a HIV vaccine in the US has found that it does not lower the risk of acquiring HIV, and in some cases may actually increase the risk of contracting the infection.

Researchers have analysed blood samples of the trial’s 2677 participants to try to assess why the vaccine, developed by Merck Research Laboratories, was not effective.

The results, which were published online by the Lancet this week, show that the vaccine did not prevent infection in those not previously infected with HIV, nor did it reduce the amount of virus in participants who became infected with HIV through exposure from an infected person while taking part in the trial.

In fact, people given the vaccine had a slightly higher risk of HIV infection. However, this increased risk was only seen in men who received the vaccine who were uncircumcised or had pre-existing immunity to adenovirus type 5, the cold virus used as a carrier for the vaccine, compared to those that received placebo.

The reasons for this increased risk are now being investigated. The researchers have stressed in their report that the vaccine itself cannot cause HIV.

Health officials, however, are remaining positive about the outcome of the study.

Larry Corey, principal investigator of the HIV Vaccine Trials Network (HVTN) said in a statement: “This trial stands as a landmark clinical HIV vaccine investigation that has profoundly informed the entire HIV vaccine research field.”

Peter Kim, President of Merck Research Laboratories, said: “As disappointing as the outcome of the study was, we will continue to follow the study participants and conduct additional analyses as part of Merck’s commitment to a comprehensive approach to address the complex challenges of the AIDS pandemic.”