American researchers said that the two field tests of an HIV/AIDS vaccine failed to explore its safety and efficacy on volunteers but may have a chance of being infected by the disease.

Both tests were stopped last September and seven other trials of similarly created AIDS have been either sopped or postponed indefinitely.

The STEP and Phambili used the vaccine made from a common respiratory virus called adenovirus type 5 that had been weakened and then loaded with fragments of HIV. Both studies were stopped after the STEP study was proven useless and may be harmful.

The working hypothesis for what went wrong is that the vaccine somehow primed the immune system to be more susceptible to HIV infection - a scenario neither foreseen nor suggested by previous studies.