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Table 2 Trade-off between increased woman-years (power) and falling adherence.

From: Microbicides Development Programme: design of a phase III trial to measure the efficacy of the vaginal microbicide PRO 2000/5 for HIV prevention

Scenario Week Adherence Observed reduction
in HIV incidence*
95% CI % power
1. Adherence is unchanged from
Week 40 to week 52
40 80% 40% (17%, 56%) 85%
  52 80% 40% (20%, 55%) 90%
  40 80% 35% (11%, 52%) 72%
  52 80% 35% (14%, 51%) 80%
  40 80% 30% (6%, 48%) 55%
  52 80% 30% (8%, 47%) 64%
2. Adherence drops to 50% at week 52 40 80% 40% (17%, 56%) 85%
  52 50% 37% (16%, 53%) 86%
  40 80% 35% (11%, 53%) 72%
  52 50% 33% (12%, 49%) 75%
  40 80% 30% (6%, 48%) 55%
  52 50% 29% (8%, 46%) 66%
3. Adherence drops to 0% at week 52 40 80% 40% (17%, 56%) 85%
  52 0% 34% (14%, 50%) 79%
  40 80% 35% (11%, 52%) 72%
  52 0% 29% (6%, 45%) 63%
  40 80% 30% (6%,48%) 55%
  52 0% 25% (2%, 42%) 50%
  1. *Compared to placebo
  2. All figures are based on following assumptions:4% HIV incidence in control arm; 85% retention at week 40; 80% retention at week 52