Maggie Fox, Washington: Trials of a new product designed to help women protect themselves from the AIDS virus were halted on Wednesday after women using it became infected at a higher rate than women not using it, researchers said.

Toronto, Canada-based Polydex Pharmaceuticals said the gel, known as a microbicide, apparently made women more vulnerable to the virus, not less vulnerable, as intended.

The microbicide, developed under the brand name Usher cell, is a cotton-based compound that had been tested in more than 500 women without any indication it raised the risk of HIV infection. It was being tested in advanced trials in 1,333 women in South Africa, Benin, Uganda and India.

A second group testing the same compound stopped its trials, too, out of concern for the women, although there was no suggestion the women in the second trial were becoming infected at a higher-than-expected rate.

"It was our hope that this product would have helped women in protecting themselves from HIV," Dr. Lut Van Damme, who was leading the trial of the Polydex product, said in a statement.

"While the findings are unexpected and disappointing, we will learn scientifically important information from this trial that will inform future HIV prevention research." It is the second spectacular failure of a microbicide a gel or a cream designed for women to use vaginally to prevent infection with HIV.

Trials of the spermicide nonoxynol-9 were stopped after it was found to raise the risk of HIV infection. The World Health Organization and the U.N. AIDS agency UNAIDS, which were helping coordinate the trial, said it was not clear why the product did not work.

"Cellulose sulfate was one of four compounds being evaluated in large-scale studies of effectiveness among women at high risk of HIV infection," WHO and UNAIDS said in a joint statement. Three other products are also in advanced trials, they said, including products based on a seaweed derivative called carrageenan.

Jeff Spieler of the U.S. Agency for International Development, which was also sponsoring the trial, said the product had appeared safe in earlier trials with fewer women.

"I am also hopeful that one or more of the other microbicide candidates now in development will be shown to be safe and effective in helping to prevent HIV infection alongwith other behavioural interventions," Spieler said in a statement.

Family Health International said it was halting its trial of the same product in 1,700 women in Nigeria. Women in both trials who become infected with HIV will be given drugs to control it, the researchers said. WHO estimates that half of the 39 million people infected with the human immuno deficiency virus today are women, and HIV is mostly transmitted through sexual intercourse between a man and a woman.

There is no cure for HIV and no vaccine against it. Condoms can prevent infection but women are often powerless to demand their use.