Several vaccine trials are being postponed or modified following the halt of Merck's experimental HIV vaccine trial. Merck in September announced that it had ended the Phase II trial, which began in late 2004 and involved HIV-negative volunteers, after the experimental vaccine failed to prevent HIV infection in participants or prove effective in delaying the progression of the virus to AIDS.

New data recently suggested that the vaccine was ineffective among some trial participants with a pre-existing immunity to a common cold virus and that the vaccine might have increased their susceptibility to HIV infection. The Merck vaccine was made from a weakened version of a common cold virus that served as a mode for providing three synthetically produced genes from HIV, known as gag, pol and nef.

After several days of discussions at an HIV Vaccine Trials Network conference last week in Seattle of Merck's trial, leaders of the trial on Monday decided to notify all of the trial's 3,000 participants whether they were given the vaccine or a placebo.

Other trials have been affected by Merck's trial because the experimental vaccines have a similar structure to Merck's vaccine. Trial participants now must be warned about the potential risks highlighted in the Merck trail if they participate in experiments that use a cold-virus carrier similar to Merck's product, Anthony Fauci, director of the National Institute of Allergy and Infectious Disease, said.