Atlanta—Pre-exposure prophylaxis (PrEP) holds the promise of significantly reducing HIV incidence among men who have sex with men (MSM) in the United States, but only if guidelines are followed across-the-board.

That’s according to a new modeling study focusing on the U.S. and published in The Journal of Infectious Diseases. It suggests that, over the next decade, use of a daily pill can drop new HIV cases by a third in the MSM cohort.

The catch, however, is that the success of a PrEP program is dependent on clinicians prescribing the medication according to federal guidelines and on patients closely adhering to the therapy.

In 2012, the FDA approved a combination of two drugs—tenofovir disoproxil fumarate and emtricitabine, marketed under the name Truvada—for daily use as PrEP to prevent HIV infection. With studies indicating that PrEP can reduce the risk of getting HIV from sex by more than 90% when used consistently, the CDC issued guidelines for the drug’s use in clinical practice in 2014.

For this study, researchers at Emory University, the CDC, and the University of Washington used network-based mathematical models of HIV transmission in a population of MSM to estimate how effectively the CDC guidelines would prevent new HIV infections.

Results indicate that, if PrEP was prescribed to 40% of MSM considered good candidates based on CDC’s guidelines and if the drugs were taken consistently, 33% of new HIV infections could be avoided in this population over the next 10 years.

Their calculation suggests that preventing one new HIV infection would require treating 25 men with PrEP for a year—which the researchers conclude is a good balance between public health impact and intervention efficiency.

“This is a modeling study of future impact, meaning that we’re providing forecasts of what could be based on the best available data that we have now,” said co-author Samuel M. Jenness, PhD, MPH, of Emory University. “There’s a big gap between the 5% of MSM using PrEP today and those who could potentially benefit from it. There’s still a lot of work to be done to promote this intervention and scale it up more broadly, but our models suggest that the CDC guidelines provide a good framework for doing so.”

Jared M. Baeten, MD, PhD, of the University of Washington notes that the mathematical model provides some valuable information.

“Every provider, public health professional, patient, and advocate who has seen the devastation wrought by HIV in the last three decades wants to see far fewer men and women presenting for care with a new diagnosis of HIV infection,” writes Dr. Baeten, who was not involved in the study. “PrEP can be a part of that outcome--especially if pragmatic approaches are sought, aimed at achieving the coverage necessary to gain population impact at scale.”

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