In a setback for a new form of HIV/AIDS prevention, scientists on Monday ended a study in Africa after a lack of success. The medication, Truvada, had shown that men who took the pill had better than 90 percent protection, a result hailed as a breakthrough for AIDS prevention. But of the 1,900 women taking Truvada or a placebo, 28 in each group had been infected by last week, according to the nonprofit group conducting the study. Previous study trials had shown better results with women, including a South Africa trial in which infection was reduced by as much as 54 percent among women who used Truvada faithfully before and after sex. Some questions remained after the study’s sudden end, such as whether the poor results were because some women responded to unpleasant side results, or because pregnant women were taken out of the study.