A once-a-day pill for preventing infection with the human immunodeficiency virus (HIV), the virus that causes AIDS, has been effective in clinical trials, and new research suggests that it works in the real world, too.

In a study conducted at sexual health clinics, participants taking the pills for so-called preexposure prophylaxis, or PrEP, had only two new HIV infections, although their rates of other sexually transmitted infections and condomless sex remained high, researchers found.

"There were a number of clinical trials that looked at PrEP and showed it to be safe and effective in that research setting, but there was little information on how PrEP would be implemented outside of those research trials," said lead author Dr. Albert Liu, of the San Francisco Department of Public Health.

Gilead's Truvada, which is a combination of two drugs known generically as tenofovir and emtricitabine, was approved by the U.S. Food and Drug Administration in 2012 as PrEP to prevent HIV infections.

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The initial trials found Truvada to be more than 90 percent effective in preventing HIV infections among gay and bisexual men and transgender women who consistently took the pill, Liu and colleagues write in JAMA Internal Medicine.

Recent results from San Francisco and the UK have also shown encouraging results.

For the new study, Liu and colleagues enrolled 557 gay and bisexual men and transgender women at two sexually transmitted infection clinics in San Francisco and Miami and a health clinic in Washington, D.C. All participants got Truvada for 48 weeks, HIV testing, counseling and checkups.

About 79 percent of participants completed the study. The researchers also took blood samples from 294 participants to determine how often they took Truvada.

Of those who gave blood samples, 80 to 86 percent had levels of Truvada high enough to say they were taking the drug at least four times per week.

Two participants were ultimately diagnosed with HIV. Both had low levels of the drug in their system.

"We did not see infections among the cohort who had higher drug levels," Liu said.

At most of the clinics, HIV infection rates had been above 2 percent, but the infection rate for the new study was lower than 0.5 percent, Liu told Reuters Health.

"I think the results of this study are very encouraging for the PrEP field, but we have much work to do," Liu said.

For example, the authors pointed out, people in Miami, and African-Americans, were less likely to have protective levels of the drug in their system.

In a research letter published in the same journal, a separate team reports that PrEP awareness and uptake among the black community is low.

Dr. John Schneider of the University of Chicago and colleagues interviewed 622 young black gay and bisexual men from the south side of Chicago in 2013 and 2014 about their PrEP awareness and use.

Of those who were not already infected with HIV, only about four percent were using PrEP. About 41 percent overall had heard of PrEP, and about 12 percent knew people who used it.

It's critical for healthcare providers and public health officials to "engage this special population in ways that (are) culturally appropriate," Schneider told Reuters Health.

William Nazareth of the Callen-Lorde Community Health Center in New York City said the new findings give insight to those outside LGBT healthcare, like religious and non-service-based community organizations.

"Those are the places that need to understand and start promoting PrEP," said Nazareth, who was not involved with the new research, but built Callen-Lorde's PrEP program.

"PrEP may not be for everyone, but it's really important that everyone learn about it," he said.