U.S. officials said on Thursday they will give up on a trial in Botswana that was trying to show whether it is possible to prevent HIV infections by taking a daily pill because too few people are being infected.

There are also problems keeping track of people enrolled in the trial, so it will be adjusted to show instead how well people can stick to the routine, the team at the U.S. Centers for Disease Control and Prevention said.

The trial of 1,200 people was trying to see if people could prevent infection with the AIDS virus if they took a daily pill that combined two HIV drugs. It was using Gilead Sciences Inc's Truvada, a combination of two drugs called tenofovir and emtricitabine.

They did not release the data on how many people in the trial became infected. The researchers also said that there appeared to be no safety concerns with the treatment so far.

The study, called TDF2, is one of several globally looking at the new approach, called pre-exposure prophylaxis, or PrEP. The idea is that a daily low dose of the drugs, which interfere with the ability of the virus to replicate, could also lower the risk of infection.

It has worked in monkeys and researchers are keen to see if it could provide an easy and cheap way to protect people from the virus, which infects 33 million globally and has killed 25 million people.

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, the CDC said in a statement.

The trial protocol and timeline will be revised to focus instead on the other remaining study questions -- primarily behavioral and clinical safety and adherence.

The problem is that new HIV infections are becoming less common in Botswana, where nearly a quarter of adults are infected.

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 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 trials are under way in the United States, South Africa, Thailand, Brazil, Peru, Kenya, Uganda and elsewhere.

Researchers are also looking for other ways to prevent infection, including by circumcising men.

There is no vaccine against the AIDS virus yet although work continues to make one, and researchers are also working on testing microbicides -- gels or creams that could be used to help prevent sexual transmission of the virus.