AIDS Vaccine Trials Halted

Researchers have vowed to push on after clinical trials of a leading HIV vaccine candidate were halted because an independent monitoring board found that the vaccine is ineffective. The vaccine is made by Merck & Co., which was funding the clinical trials along with NIH's National Institute of Allergy & Infectious Diseases (NIAID).

The Merck vaccine, known as V520, was considered one of the most promising in development. "We share the disappointment of the research and HIV communities," said Peter S. Kim, president of Merck Research Laboratories, when disclosing the outcome. "Sadly, developing an effective AIDS vaccine remains one of the most challenging tasks facing modern medicine." Groups including the International AIDS Vaccine Initiative and the AIDS Vaccine Advocacy Coalition expressed similar views but encouraged researchers to keep working.

"It's important that we not equate the failure of one product with failure of a concept," says Margaret I. Johnston, director of the vaccine research program in NIAID's AIDS Division. Several classes of vaccines are in development, and V520 is among about 30 individual vaccines being studied. Instead, Johnston says, it's prudent to learn from the Merck trial to improve future studies. According to Kim, Merck will look at the trial data closely and share them with other vaccine efforts.

V520 consists of gag, pol, and nef HIV genes delivered in a type 5 adenovirus vector. Designed to stimulate a CD8 T-cell-based immune response, the vaccine didn't prevent infection or reduce the amount of virus in people who did become infected. In 2003, VaxGen, a now nearly defunct South San Francisco-based biotechnology company, reported that its subunit vaccine, using the gp120 HIV surface protein to generate an antibody response, failed to show efficacy in Phase III trials.

Researchers continue to take a variety of tacks to generate T-cell, antibody, or combined responses. "Antibody vaccine approaches are an absolute top priority," Johnston says. "Most people believe we will need a broadly neutralizing antibody to prevent infection. No one ever considered that these T-cell vaccines would be a final goal, but maybe an interim measure until we crack the antibody problem."