A new, highly sophisticated mathematical modeling study projects that widespread use of Truvada (tenofovir disoproxil fumarate/emtricitabine) as pre-exposure prophylaxis (PrEP) among adult men who have sex with men (MSM) would yield a major reduction in the rate of new HIV infections (known as incidence) in the overall MSM population as well as the number of MSM living with the virus (known as prevalence). The study also found that even if the adolescent MSM population did not take PrEP, they would still benefit considerably from adult MSM’s use of the prevention modality, both in terms of reduced HIV incidence and prevalence.

Additional widespread coverage of PrEP among adolescent MSM would not, however, provide a considerable additional reduction in HIV incidence and prevalence in the overall MSM population. That said, providing PrEP to this relatively small population would not significantly reduce the efficiency of Truvada’s capacity to prevent infections on a public health level, measured as the number of cumulative years MSM need to spend on PrEP to avert one new infection.

The study, which was published in PLOS ONE, relied on two published models of HIV transmission among adults and adolescent MSM. The researchers simulated an HIV epidemic among 13,500 theoretical MSM 13 to 39 years old (500 males for each one-year age) in which no one used PrEP. Then they compared the outcome of this simulation with a simulated epidemic in which only MSM 19 to 39 years old used PrEP and another epidemic in which adolescents 16 to 18 years old also used PrEP.

In the no-PrEP-use scenario, 23.2% of the sexually active population had HIV, and there were an annual 322.7 new infections per 10,000 at-risk men over a 10-year period.

If 40% of the adult MSM who met the Centers for Disease Control and Prevention guidelines indicating they were good PrEP candidates received Truvada for prevention, this meant that there were an average of 3,760 men on PrEP at any time. (These men were projected to adhere to the daily drug regimen at varying rates, resulting in varying degrees of PrEP efficacy.) In this scenario, the HIV prevalence among all MSM in the cohort, including the non-PrEP-using adolescents, would decline to 17%, a 26.7% reduction compared with the no-PrEP-use scenario. The prevalence of HIV among 18-year-old MSM would decline from 6% to 4.3%.

In the scenario in which only adults used PrEP, HIV incidence per 10,000 people at risk would decline to 206.4 new cases per year. This translates to 693 averted infections per 100,000 men at risk for infection per year. The number of people needed to take PrEP for one year to avert one infection was 33.

Over a 40-year period, the adult-PrEP-use scenario would reduce HIV prevalence to just 7.4% and incidence per 10,000 at-risk men from 311.9 cases to 133.1 cases annually. HIV prevalence among 18-year-old MSM would decline to 3.8%.

Counterintuitively, providing PrEP to 10% of adolescent sexual minority males (ASMM), in addition to the 40% coverage in adult MSM, would result in one fewer infection averted per year per 100,000 at-risk MSM in the overall population, or a 0.1% decrease in the percentage of infections averted. In essence, this finding indicates that that level of PrEP coverage in the adolescent population would not have a significant effect on HIV infections. Note that the researchers estimated that adherence to PrEP would be poorer among adolescents compared with adults.

Doubling the adolescent PrEP coverage from 10% to 20% resulted in an increase in the percentage of infections averted from 28.9% to 29.7%. Pushing the coverage to 30% increased the percentage of infections averted to 29.9%. The overall number of MSM in the entire population, adolescent and adult, needed to receive PrEP for one year to avert one new case of HIV in this 30%-adolescent-use and 40%-adult-use scenario increased very modestly, from 33 to 34, indicating that Truvada’s use as prevention remained highly efficient.

“Even when ASMM are not specifically targeted for PrEP uptake,” the study authors concluded, “adult PrEP can be an effective intervention for ASMM, reducing prevalence in this group by as much as 29% over 10 years, preventing transmissions of HIV from young adult MSM to ASMM by disrupting an essential mechanism for the persistence of the epidemic. The impact of age mixing on HIV acquisition risk, particularly among the young, has been demonstrated in numerous context and populations. It has also been shown that young people in age-discordant relationships are more likely to engage in higher-risk behavior.”

To read the study, click here.