Model Indicates Broad Use of PrEP Could Lead to Marked Reductions in HIV Incidence Among MSM

By Eric Brus

There is an extensive and growing body of evidence indicating that pre-exposure prophylaxis (PrEP) is highly effective in preventing HIV infection among gay, bisexual, and other men who have sex with men (MSM). However, only limited information has been available on the projected impact that the extensive rollout of PrEP could have on new HIV infections among high-HIV-incidence groups, such as MSM.

In a new study, researchers describe their use of a mathematical model to estimate the impact that the rollout of PrEP, based on the behavioral indications in CDC's PrEP guidelines, could have on HIV incidence in U.S. MSM. Other variables considered in the model were the percentage of MSM with indications for PrEP who receive treatment and their rate of adherence to the PrEP regimen.

The researchers used a model of HIV transmission dynamics among MSM to estimate the proportion of HIV infections averted (PIA), the number of persons needed to treat with PrEP to prevent one new infection, and related epidemiological outcomes after implementing PrEP according to the behavioral indications in CDC's PrEP guidelines.

The researchers developed a base scenario in which 40 percent of MSM with behavioral indications started PrEP and about 62 percent of those receiving PrEP were highly adherent. Under this scenario, the model projected that, over the next decade, the use of PrEP by U.S. MSM would avert an estimated 1,162 infections per 100,000 person-years. This is equivalent to one-third (33 percent) of expected infections among MSM.

The model also predicted that the number of persons needed to treat (NNT) with PrEP to prevent one HIV infection would be 25. The researchers also used to model to calculate the expected impacts of other scenarios for PrEP coverage and adherence. They found that increasing PrEP coverage and adherence jointly raised the PIA, but that reductions to the NNT were associated with better adherence only.

"Implementation of CDC PrEP guidelines would result in strong and sustained reductions in HIV incidence among MSM in the United States," the researchers concluded. "The guidelines strike a good balance between epidemiological impact (PIA) and efficiency (NNT) at plausible scale-up levels. Adherence counseling could maximize public health investment in PrEP by decreasing the NNT."

Eric Brus is the Director of Health Information at AIDS Action Committee. This report is produced by the Health Library of the AIDS Action Committee in collaboration with the New England AIDS Education and Training Center Minority AIDS Initiative Project. The full version is available here.