PrEP Math

A study forecasts the difference an HIV prevention tool might make

by Chael Needle

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PrEP (pre-exposure prophylaxis) is fairly new, but the HIV prevention tool is not yet having a huge impact on public health. According to the CDC, one in three primary care physicians (PCPs) and nurses have not heard of PrEP. Adding to the slow uptake is the fact more clarity is needed about who might be a candidate for PrEP and what increased coverage of PrEP might mean for our comprehensive HIV prevention efforts.

A new mathematical-modeling study, “Impact of the Centers for Disease Control’s HIV Preexposure Prophylaxis Guidelines for Men Who Have Sex With Men in the United States,” by Jenness et al and published recently in The Journal of Infectious Diseases, provides insight into the potential impact of PrEP among sexually active gay and bisexual men, and other men who have sex with men (MSM). Researchers assessed impact in relation to numbers of infections averted and the number needed to treat based on the behavioral indications of the CDC’s PrEP guidelines and the scaling-up of coverage.

The results show that one-third of new infections in the U.S. could be prevented among MSM over the next ten years, with forty-percent PrEP coverage among at-risk individuals and sixty-two percent adherence. The study also showed that increased coverage and adherence together increases the percentage of infections averted. The number needed to treat, however, is only reduced through better adherence. The open-access article, which includes many more intricacies than can be covered here, concludes: “Implementation of CDC PrEP guidelines would result in strong and sustained reductions in HIV incidence among MSM in the United States.”

These guidelines, which cover other population categories, such as heterosexuals and injection-drug users, define “substantial risk” for MSM as recent condomless anal sex (receptive or insertive) with a partner whose HIV serostatus is unknown within or outside of a monogamous relationship, or anal sex, with condoms or condomless, in an ongoing serodifferent relationship.

Notes Samuel M. Jenness, PhD, Department of Epidemiology, Emory University, “It seems like those who are [prescribing PrEP] have a number of different interpretations of who among their patient population, or client population, is going to be the best candidate for PrEP. And so that was really the motivation for the CDC and Health Services for releasing the clinical practice guidelines for how risk assessment should be conducted and how patients should be managed in terms of getting on PrEP, if there are indications for it. Our study was the first to date that looked at the future impact of HIV incidence and the number of infections that we can avert, or prevent, over the next decade if clinicians followed those guidelines.

“We were specifically interested in the individual behaviorial components of those guidelines [analyzed through different scenarios involving monogamy and non-monogamy, and serodiscordancy]….So, our paper looked at the individual implications of how the behaviorial guidelines could be tied into PrEP indications and, if clinicians followed them, what sort of HIV prevention impact could be made.”

Dr. Jenness and his team assessed how those guidelines might be interpreted in clinical practice, realizing that some interpretations might depart from a literal assessment and seeking “to understand whether there would be a differential impact of interpreting them one way versus the other.” He continues: “We saw some differences…[but] at the end of the day, we’re talking about a relatively small difference if they interpreted them literally or more realistically.

“The main finding coming out of the study is that we can prevent up to a third of infections if the guidelines are followed. If they were interpreted in a different way that might fall down to roughly twenty percent, so it’s not a major difference,” he notes.

As for whether, based on the CDC guidelines, PrEP would have a substantial impact, Dr. Jenness notes: “I think you could take it in either direction because, on the one hand, a third of new infections prevented would be a substantial gain, and drive the incidence down among MSM in quite a significant way, but, on the other hand, that means there are two-thirds of infections that are still going on….We know that somewhere around five to ten percent of men who are indicated [for PrEP] currently are using PrEP, and so that’s going to require significant scaled-up efforts to close that gap between current use and what we’re using as our base scenario in the model. So, there’s been some discussion about whether there should be supplemental targeting techniques on top of the CDC guidelines and we’re interested in that too,” he says about the substance of a planned follow-up study “to understand ways that we might use other features of gay and bisexual men’s sexual risk histories or other sexual network charcteristics to identify other high-risk men who would be good candidates for PrEP but that aren’t being adequately identified under the current guidelines.”

Dr. Jenness explains: “Currently the guidelines are focused on very specific anal intercourse acts and the types of partnerships that those occur in and the motivation for that is to cast a wide net to incorporate men who exhibit some of those risk behaviors. Still at the same time [assessments] may be missing out on broader community and partnership-level risk factors that aren’t captured at that individual level. Sexual trauma [as you mentioned] might be one of them. Demographics might be another.

“We know that, for example, black men who have sex with men have a much higher risk of becoming infected compared to white men, and, although our study didn’t look at the racial disparities in PrEP uptake or do projections by race, there might be some utility down the road of doing supplemental targeting for populations or subgroups that have known higher incidence to begin with, regardless of whether they are exhibiting these particular behavioral indications.”

Says Dr. Jenness about the article: “The key target group that we’re aiming for here is people who are working on designing PrEP scale-up programs because that’s for both clinicians and community members, MSM, who would benefit from PrEP, public health agencies, other AIDS service organizations who need to make decisions about how PrEP fits into their comprehensive HIV prevention plan. It’s very expensive and there’s a whole mix of coverage actions in terms of insurance and payment, and it’s not always the right choice for every man and he needs to make decisions about whether it is for himself.”


Chael Needle wrote about the UN’s recent political declaration on HIV/AIDS in the July issue. Follow him on Twitter @ChaelNeedle.