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.
Among those participants who became infected, vaccination was not associated with an effect on viral load; similar levels of circulating virus were detected among vaccine and placebo recipients.
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.
"If T-cell immunity is critical to produce protection against HIV, the findings from Step suggest that future candidate vaccines must elicit responses that are more broadly reactive or qualitatively different from the immune responses elicited by the Merck vaccine candidate in this trial," The Lancet quoted Susan Buchbinder, M.D., San Francisco Department of Public Health, as saying.
"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.