An international clinical trial of an HIV vaccine has been halted prematurely. The Merck & Co., Inc. vaccine V520 was considered to be one of the most promising vaccines in development, but it has failed to block or slow down HIV infections in high-risk volunteers.

V520 offered a new approach over existing, ineffective vaccines, which focused on inducing the immune system to produce antibodies. Instead, V520, which comprises a weakened adenovirus loaded with three HIV genes, stimulated a T-cell response. “The concept was that if someone getting the vaccine is later exposed to HIV, the immune system would recognize those HIV proteins and go after the virus,” said Keith Gottesdiener, a senior research executive at Merck & Co., Inc. ( Reuters , 21 September 2007).

Although the vaccine was not expected to prevent HIV infection, it was hoped that it would inhibit the growth of the virus enough to delay the onset of AIDS, and allow more time to develop an effective therapy. However, similar numbers of volunteers contracted HIV, whether they received V520 or a dummy vaccine. V520 also failed to reduce virus levels in the blood of infected individuals ( Scientific American , 25 September 2007).

“It is a huge disappointment because this vaccine has shown promise all the way through, but it's only when you get in on these big trials that you start to see how the vaccine behaves,” said Glenda Gray, one of the principal investigators of the clinical trial ( AFP , 22 September 2007).

But Mitchell Warren, executive director of the AIDS Vaccine Advocacy Coalition, had a positive outlook: “The Merck vaccine was a step along the way that will give us clues [to] how we can better trigger the immune system with other vaccines.” (Scientific American, 21 September 2007.)