Tuesday, March 22, 2016

PrEP Adherence Patterns Strongly Impact Individual HIV Risk & Observed Efficacy in Randomized Clinical Trials

Randomized controlled trials (RCT) suggest that the efficacy of tenofovir-based pre-exposure prophylaxis (PrEP) strongly depends on consistency of PrEP use. We explore how patterns of pill-taking and waning of PrEP protection may affect PrEP efficacy for HIV prevention.

A two-arm RCT was simulated by mathematical models assuming that prescribed daily doses were skipped periodically, randomly or in large blocks. Risk-driven adherence, in which PrEP was taken when sex was expected, was also investigated. Three temporal PrEP protection profiles were explored: long (5 days), intermediate (3 days) and short (24 hours). Modeling results were compared to the efficacy observed in completed RCTs.

Expected PrEP efficacy was 60% with periodic, 50% with random and 34% with block adherence when PrEP had a long protection profile and pills were taken only 50% of the days. Risk-driven pill-taking resulted in 29% and 37% daily pills taken and efficacy of 43% and 51% for long protection. High PrEP efficacy comparable with that observed in Partners PrEP and CDC Botswana trials was simulated under long protection, high overall adherence and limited block pill-taking; the moderate efficacy observed in iPrEx and Bangkok trials was comparable with the 50% adherence scenarios under random pill-taking and long protection.

Pill-taking patterns may have a substantial impact on the protection provided by PrEP even when the same numbers of pills are taken. When PrEP retains protection for longer than a day, pill-taking patterns can explain a broad range of efficacies observed in PrEP RCTs.

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  • 1Vaccine and Infectious Disease Division, Fred Hutchinson Cancer Research Center and Department of Applied Mathematics, University of Washington, Seattle, Washington, USA 
  • 2 Sainte-Justine Research Centre, University of Montreal, Montreal, Quebec, Canada 
  • 3 Vaccine and Infectious Disease Division, Fred Hutchinson Cancer Research Center and Department of Global Health, University of Washington, Seattle, Washington, USA.
  •  2016 Mar 16. 

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