It is not clear why the gel did not work, as it had a surprising success rate in a previous trial, The New York Times reported.
Researchers have been working to find a vaginal gel that protects women against the virus that causes AIDS but still allows them to become pregnant and can be used secretly by women who fear being refused or beaten if they ask their sexual partner to wear a condom.
The first trial, called Caprisa, which concluded in the summer of 2010, found a vaginal gel containing the drug tenofovir protected 30 percent of women who used it, and women who used it most regularly reduced their chances of infection by 54 percent.
Voice, the second trial, began in 2009 and enlisted more than 5,000 women in South Africa, Uganda and Zimbabwe. It was divided into three parts, comparing three different products against a placebo. The gel was tested again, as well as a tenofovir pill and a Truvada pill, which contained tenofovir and a booster drug.
The trial for the tenofovir pill was canceled in September because it did not appear to be working.
"Even when we have more information available to us, understanding why our results differed from the Caprisa results may not be clear," said Sharon L. Hillier, a lead researcher for the Microbicide Trials Network.
In a statement released by Hillier and Dr. Ian McGowan, another researcher from the network, the two speculate the trial may have been unsuccessful because too few women used the gel regularly.
The trial of the Truvada pill is continuing and is expected to run until mid-2012 and be released in 2013, The New York Times reported. Several other trials of the gel in different formations and dosing are planned or are currently under way.
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