This is a big blow to the drugmaker, which has been chasing an HIV vaccine for two decades and regularly boasted of its commitment to combating AIDS. But the trial was ended after an independent safety monitoring board decided the vaccine was ineffective and would fail to meet mid-stage trial goals. The study was designed to determine whether the vaccine prevented HIV and reduced the amount of virus in those who developed infection.

“We share in the disappointment of the research and HIV communities today. Sadly, developing an effective AIDS vaccine remains one of the most challenging tasks facing modern medicine,” says Peter Kim, Merck’s chief scientist, in a statement. Adds Glenda Gray, the principal investigator of the trial, which was sponsored by the HIV Vaccine Trials Network and which took place in South Africa: “This is a huge disappointment for all of us who have been involved in the search for an HIV vaccine.”

[UPDATE: This apparently spells the end of Merck's work into HIV vaccines, at least for now. Pharmalot just spoke with two Merck execs, who told us: "We're not sure yet what our next step will be because the data is still new," says Keith Gottesdiener, a vp who heads clinical infectious diseases and vaccines." And Mark Feinberg, vp, medical affairs and health policy for vaccines, adds that "we would be very interested if there were a clear path forward. At present, it's not clear what the most promising path forward will be, based on these results."]

The study was a multicenter, randomized, double-blind, placebo-controlled phase II test-of-concept clinical trial, and the trial enrolled 3,000
HIV-negative volunteers from diverse backgrounds between 18 and 45 years of age at high risk of HIV infection. The study evaluated two primary endpoints: whether the vaccine prevented HIV infection and whether the vaccine reduced the amount of virus in those who developed the infection.

The Merck adenovirus-based vaccine used a cell-mediated immune response approach on operated on the theory that HIV genes in the vaccine would stimulate the body to generate an HIV-specific immune response through the body’s own CD8 T-cells, which become programmed to recognize and kill HIV infected cells.

According to Merck’s statment, the vaccine didn’t prevent infection: in volunteers who received at least one dose of the three-dose vaccine series, 24 cases of HIV infection were observed in the 741 volunteers who received vaccine and 21 cases of HIV infection were observed in the 762 participants in the placebo group. In the subgroup who had received at least two vaccinations and who were HIV negative for at least the first 12 weeks of the trial, 19 cases of HIV infection were observed in the 672 volunteers who received vaccine and 11 cases were observed in the 691 volunteers who received placebo. In addition, the vaccine didn’t reduce the amount of virus in the bloodstream of those who became infected; HIV RNA levels approximately 8 to 12 weeks after diagnosis of infection were similar in the vaccine and the placebo arms.