A vaginal gel researchers had hoped would help prevent HIV transmission for women is ineffective, according to a report on Tuesday.
Researchers leading the Tenofovir gel study at 15 sites across South Africa, Uganda and Zimbabwe recommended that 5 000 women who participated in the trial should stop using it, The Times reported.
The results of this second study shocked scientists when they proved the gel, developed by researchers at the University of KwaZulu-Natal, could not save millions of lives.
Last year, the Centre for the Aids Programme of Research in SA found the gel was 39% successful in preventing HIV infection.
Director of the Aids research centre Professor Salim Abdool Karim said he was "baffled" by the results of the new study.
"This was totally unexpected because there was good evidence from laboratory research, animal studies and human trials, which showed that Tenofovir gel prevented Aids," he was quoted as saying.
He and his wife, Quarraisha Abdool Karim, were expecting to have the gel in clinics next year, but the new findings sent them back to the drawing board.
He said it was not clear whether the new result was due to inadequate use of the gel by women participating in the study, insufficient drug levels in the women at the time of HIV exposure, or some other reason.
The couple was widely lauded after the gel was presented at the International Aids Conference in Vienna last year - where the global microbicide research community agreed a second study was needed to confirm the protective effects of the gel.
The study saw participants, aged between 18 and 30, using the gel 12 hours before intercourse and within 12 hours after intercourse.
Funders included amongst others, the science and technology department, the US government and the Bill and Melinda Gates Foundation.