"Cellulose sulfate" is another microbicide found ineffective in preventing the transmission of HIV. On February 1, 2007, the International AIDS Society announced that two phase III trials of cellulose sulfate had been stopped because preliminary results suggested a potential increased risk of HIV in women who used the compound. There is no satisfactory explanation as to why application of cellulose sulfate was associated with a higher risk of HIV infection than placebo. According to a review of microbicide drug candidates by the World Health Organization on March 16, 2007, a large number of compounds (more than 60 in early 2007) are under development; at the beginning of that year, five phase III trials testing different formulations were underway.