The results of the worldwide trials prompted scientific inquiry as researchers try to assess what happened and whether they should known the outcome, The Washington Post reported Friday.

Both field tests were halted in September and seven other trials of similarly designed AIDS vaccines have been stopped or postponed indefinitely.

"This is on the same level of catastrophe as the (NASA space shuttle) Challenger disaster," said Robert Gallo, co-discoverer of the human immunodeficiency virus, the virus that causes AIDS, and head of the Institute for Human Virology in Baltimore.

The STEP and Phambili studies used the same vaccine and were stopped when it became evident the studies were futile and possibly harmful, the Post reported.

One hypothesis for what went wrong is the vaccine somehow prepared the immune system to be more susceptible to HIV infection, a scenario that wasn't foreseen or suggested by previous studies, the Post said.

The National Institutes of Health in Bethesda, Md, which funded the trials, will meet week to reassess its AIDS vaccine program.