An analysis of 32 studies of Truvada (tenofovir/emtricitabine) as pre-exposure prophylaxis (PrEP) found that almost all of those who contracted HIV had undetectable or very low levels of the drug in their bodies at the time. Researchers from Gilead Sciences, Truvada’s manufacturer, presented the findings from their analysis of these studies at the ASM Microbe 2016 conference in Boston.

The investigators in the trials conducted dried blood spot tests on participants to determine levels of tenofovir in specific blood cells at the time they contracted HIV. These tests can detect adherence to Truvada with a window looking back about eight weeks.

Thirty of the 32 studies began after the July 2012 U.S. approval of PrEP.

Participants lived in 16 countries and five continents.

This analysis looked at only those who received Truvada on an open-label basis (meaning they knew they were receiving the drug and not a placebo) and excluded those who received Truvada on a deferred basis or were in observation-only arms of studies.

The studies had a cumulative 8,478 participants, ranging from 23 to 1,225 per study, including 7,002 men, 1,378 women and 76 transgender women. The participants were followed for a cumulative 7,061 years.

Sixty-seven of the participants contracted HIV during the studies, for an overall infection rate of 0.95 percent per year. The highest seroconversion rate, 7.7 percent per year, was seen among men who have sex with men (MSM) 18 to 25 years old.

Sixty-four of those who became HIV positive were men (1.03 percent per year infection rate), all of whom were MSM; two were women (0.25 percent per year); and one was a trans woman (2.07 percent). The average age of those who seroconverted was 25, with a range between 17 and 49 years old.

Six of the seroconversions took place after the end of the treatment phase of a study.

Dried blood spot test results were available for 32 of those who seroconverted. Seventeen of them showed undetectable drug levels, and 14 of the remaining 15 showed levels consistent with taking fewer than two tablets of Truvada per week. That leaves one person who was apparently taking two or more tablets per week, but not taking Truvada daily.

Research has suggested that taking four to seven tablets of Truvada per week provides maximum protection against HIV. There has been one documented case, identified in a private medical practice, of someone contracting the virus while apparently adhering well to PrEP.