On Thursday, CDC officials said a Botswana trial studying whether the drug Truvada (tenofovir and emtricitabine) can be taken daily in low doses to prevent HIV infection will shift gears, as incident infections among its 1,200 participants were too few to determine efficacy. The "TDF2" trial is one of several ongoing studies worldwide testing pre-exposure prophylaxis (PrEP) using antiretrovirals, a method that has been found to work in monkeys. The Botswana study will now focus on safety and treatment adherence, the agency said.

"The TDF2 study will be adapted due to unanticipated challenges that make it very unlikely that the trial will be able to determine if tenofovir-emtricitabine is effective in reducing the risk of HIV infection," officials said.

"While the trial met its original enrollment goals, this study will not be able to determine efficacy given much lower than anticipated HIV incidence in the study population (likely due to declining HIV rates in Botswana generally, and to extensive HIV prevention services provided to all participants), and challenges in retaining participants in this highly mobile population of young adults," the agency said.

"The trial protocol and timeline will be revised to focus instead on the other remaining study questions - primarily behavioral and clinical safety and adherence," CDC said. "The trial, however, will provide critical information on safety and adherence to help guide potential implementation planning should PrEP prove effective in other trials."

Other, similar studies are continuing in countries including the United States, South Africa, Thailand, Brazil, Peru, Kenya, and Uganda.