A clinical trial testing the effectiveness of what was believed to be a promising new HIV vaccine has resulted in failure. The vaccine, made by the pharmaceutical company Merck, offered no protection against infection by the Aids virus.
The Step Study showed that the vaccine did not prevent infection in those not previously infected with HIV, nor did the vaccine reduce the amount of virus in those study participants who became infected with HIV through exposure from an infected person while in the trial.
Researchers also found that the likelihood of becoming HIV positive was highest among men who received the vaccine, compared to those that received placebo.
In the comprehensive analyses of immune responses to the vaccine, the researchers analyzed study participants' blood samples to try to assess why the vaccine did not prevent infection and why some study participants who received the vaccine were more likely than others to develop HIV infection.
The vaccine was effective at producing an immune response: 77 percent of those vaccinated who later developed HIV infection while in the study had generated HIV-specific T-cells prior to infection.
Also, among those who received the vaccine, no major differences were found in the HIV-specific immune responses in those who developed HIV infection during the trial compared to those who did not.
The researchers believe that these findings mean that this type and level of production of an HIV-specific T-cell immune response alone may not be sufficient to prevent infection.
"It remains possible, too, that immune responses produced by T-cell based vaccines alone may not be sufficient to protect against HIV infection or disease. We will continue to explore all results of the Step study to help inform the continued search for a vaccine," she added.
The study is published in an online edition of The Lancet medical journal.