In April 2011, the FEM-PrEP study found that giving HIV-negative women tenofovir/FTC (Truvada) pills to prevent their acquiring HIV was totally ineffective: there was no difference in HIV incidence between women taking Truvada and women taking placebo.
In July 2011, however, the Partners PrEP study found that Truvada was 73% effective in preventing HIV transmission between heterosexual partners of different HIV status.
How do we explain why giving HIV-negative women antiretroviral pills made no difference to the HIV infection rate in one trial, but prevented at least two in every three infections in the other? The difference, it appears, is that in the Partners PrEP trial, adherence to the study medication was very high, whereas in FEM-PrEP, despite counselling and support, less than half the women took their PrEP pills regularly.
The Partners PrEP study
The Partners PrEP study enrolled 4758 serodiscordant couples in Kenya and Uganda; the HIV-negative partner was female in 38% of couples. This study had three arms: a daily tenofovir pill, a daily Truvada pill, or placebo.
There were 17 infections in participants on tenofovir, 13 on Truvada and 52 on placebo. Efficacy overall was 75% in those assigned Truvada and 67% in those assigned tenofovir, though confidence intervals (44% to 81% in tenofovir and 55% to 87% for Truvada) overlapped, so the efficacy of the two regimens was the same statistically. The same was true of efficacy observed in women (65%) and men (70.5%).
Adherence according to pill counts of unused medication was 97%. A substudy (Donnell) compared tenofovir levels in the blood of 29 out of the 30 people who became infected in the two PrEP arms with levels in a random selection of 198 people who did not become infected.
Tenofovir was undetectable in the blood of 70% of the people who became infected but only 18% of the people who did not, indicating a ‘true’ adherence level of about 80% – and having a detectable level of tenofovir in the blood was associated with an 86% reduction in HIV risk in those taking tenofovir and a 90% reduction in those on Truvada.
The FEM-PrEP study
In the FEM-PrEP study, 2056 HIV-negative women in South Africa, Kenya and Tanzania were randomised to take a daily Truvada pill or a placebo. The trial was stopped when an interim analysis found near-identical HIV infection rates in both trial arms. There were 33 HIV infections in women taking Truvada and 35 in women taking placebo; this translates into annual incidence rates of 4.7% and 5.0% respectively. This 0.3% difference is no difference at all, statistically speaking (hazard ratio 0.94, 95% confidence interval 0.59 to 1.52, p = 0.81).
Participants in the study said they took their pills 95% of the time and adherence as measured by pill count was 85%. However when drug levels of tenofovir and FTC were measured in the blood of women assigned to Truvada, the investigators found that less than 50% of the women who should have been taking the drug had actually done so in the last 12 days, and less than 40% within the last 48 hours.
In infected participants, 26% had detectable levels of tenofovir in their blood in the last visit before they tested HIV positive, 21% at the visit they tested positive, and 15% at both visits; in non-infected participants whose samples were taken at the same visits they were 35%, 38% and 26% respectively.