Hundreds of AIDS vaccine volunteers in South Africa are being warned that an experimental vaccine they received may make them more likely to contract HIV, as opposed to helping to prevent infection (Washingtonpost.com, 10/24).

Researchers began contacting the 801 vaccine trial participants on Tuesday, telling them that the vaccine might have actually increased their risk of contracting HIV. While the vaccine can’t cause infection, it could have created changes in the immune system that would make later infection easier, say researchers.

“This is my worst nightmare,” said the lead South African researcher, Glenda Gray. “I haven’t slept for days. I have a headache. I’m ready to resign from trials for the rest of my life.”

The decision to contact the participants comes after Merck & Co. was forced to halt a trial for its promising AIDS vaccine last month after it was determined that the vaccine didn’t protect study participants from HIV infection.