JOHANNESBURG, Oct. 24 -- South African AIDS researchers have begun warning hundreds of volunteers that a highly touted experimental vaccine they received in recent months might make them more, not less, likely to contract HIV in the midst of one of the world's most rampant epidemics.

The move stems from the discovery last month that an AIDS vaccine developed by Merck & Co. might have led to more infections than it averted among study subjects in the United States and other countries. Among those who received at least two doses of the vaccine, 19 contracted HIV compared with 11 of those given placebos.

Researchers shut down the trial on the grounds that the vaccine was proving ineffective, but the surge in infection among vaccinated volunteers prompted intense scientific debate and anxiety among researchers. The failure of the Merck vaccine is the latest in a series of disappointing results for research projects aimed at curbing AIDS. http://www.washingtonpost.com/wp-dyn/content/article/2007/10/24/AR2007102402514.html