The first microbicide candidate to reach the final phase of testing has failed to prevent HIV transmission, researchers announced on Monday.

Testing of the microbicide, Carraguard, was carried out over a three-year period on 6,000 women in South Africa, and was completed in March 2007. But there was no difference in HIV infections between women in the group using Carraguard compared to the placebo group.

"The trial ... was unable to demonstrate Carraguard's efficacy in preventing HIV transmission," noted Dr Khatija Ahmed, principal investigator of the trial.

The microbicide developed by the Population Council, an international non-profit organisation, contains carrageenan, which is derived from seaweed and widely used in the food and cosmetics industries as a gel, stabiliser and thickening agent.

Laboratory, animal and early human tests suggested it might prevent HIV and other sexually spread infections, but Ahmed admitted that "what Carraguard showed in the lab couldn't be converted to humans".

She suggested that the low adherence rate could have been a factor: women who participated in the study used Carraguard less than half the number of times they had sex, and only 10 percent said they used it every time as directed. However, condom use shot up from 33 percent when the study began, to 64 percent.

While acknowledging that the news was a disappointment, Ahmed stressed that the Carraguard trial was a "major milestone for microbicide development", as the trial had been completed with no safety concerns being raised.

This is another setback in the race to develop an effective microbicide - applied via a range of products like gels, films and sponges - that could help women prevent the transmission of HIV and other sexually transmitted infections.

In 2000, a large full-scale trial showed that nonoxynol-9, a potential microbicide, was unsafe after women in the study developed a higher risk of HIV infection.

Seven years later, microbicide research was dealt another blow when the US-based reproductive health research organisation, CONRAD, announced the premature end of trials of a cellulose sulphate-based microbicide after the data safety and monitoring committee found a higher number of infections in the active group compared to the placebo group.

Advocates and researchers are reluctant to describe this trial as a setback. Fiona Scorgie, programmes coordinator at the Gender AIDS Forum, a non-governmental organisation monitoring microbicide trials in South Africa, told IRIN/PlusNews that although the end result had been disappointing, the trial had been "successful on another level".

The women participating in the trials had benefited from regular health screenings, while the safety of Carraguard meant that it could be used in future microbicide trials as a "vehicle for more specific substances, like antiretrovirals", but further development was needed.

According to Scorgie, communities also had to be involved in the process, rather than being passive recipients. "Communities have a very important role to play ... it's important that we inform ourselves and remain critical".