Background: Recent trials of pre-exposure prophylaxis (PrEP) against HIV infection have generated conflicting efficacy data. Objective measures of adherence are critical for interpreting trial data and may help explain the variability in efficacy results across trials. Methods: The Partners PrEP Study (“the parent trial”) is a placebo-controlled trial of oral tenofovir and emtricitabine/ tenofovir PrEP among 4758 HIV-1 serodiscordant couples in East Africa. Adherence was measured through clinic-based pill counts and self-report on a monthly basis. An ancillary adherence study (AAS) also measured adherence with unannounced home-based pill counts (UPC; conducted every month for the first 6 months and quarterly thereafter) and the Medication Event Monitoring System (MEMS; ongoing recordings of bottle openings) at 3 sites in Uganda. Sociodemographic data were collected at enrollment. Results: A total of 1147 HIV-uninfected partners were enrolled in the AAS at the time parent trial efficacy data were released (July 2011): 47% were female, median age was 34 years (IQR 30-40), median years of education were 6 (IQR 3-7), 98% were living with their HIV-infected partner, the median duration of partnerships was 8.5 years (IQR 3.7-15.3), and 29% reported having unprotected sex within the month prior to enrollment in the parent trial. Enrollment in AAS and the parent trial occurred concurrently in 25% of couples; the remainder started the AAS 1-21 months after randomization in the parent trial. Median AAS follow-up was 11 months (IQR 7-15). Median adherence among AAS participants was 98.8% (IQR 96.0- 99.8) by clinic-based pill counts, 99.4% (IQR 98.0-100.0) by self-report, 99.1% (IQR 96.9-100.0) by UPC, and 92.1% (IQR 85.9-94.2) by MEMS. Conclusions: Adherence in this subset of participants in the Partners PrEP Study was high by multiple measures, thus providing confidence in the efficacy data from the trial. The lower adherence levels seen by MEMS likely reflect device nonuse.
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