How We Know HIV PrEP Works and Why Adherence Changes the Number

The headline that pre-exposure prophylaxis is about 99 percent effective is accurate, but it describes a specific condition: the medicine being present in the body at the moment of exposure. The Centers for Disease Control and Prevention states that oral and injectable PrEP reduce the risk of getting HIV from sex by about 99 percent when taken as prescribed, and that oral tenofovir reduces risk from injection drug use by at least 74 percent. Both figures are adherence-dependent. The history of how researchers measured PrEP efficacy is largely the history of learning to separate whether a drug works from whether people actually take it.

Efficacy versus effectiveness, and why the gap is adherence

The first large randomized placebo-controlled trials of oral tenofovir-based PrEP reported what looked, at first glance, like modest protection. In the iPrEx trial, published in 2010, daily emtricitabine-tenofovir among men who have sex with men and transgender women reduced HIV acquisition by about 44 percent compared with placebo in the intention-to-treat analysis. That single number understated the drug, because roughly half of participants had no detectable study medication in their blood when tested. Among participants with detectable drug, the estimated reduction was about 92 percent, and pharmacology substudies linked intracellular tenofovir concentrations consistent with four or more doses a week to high protection. The molecule was potent. The variable was the pill-taking.

The pattern repeated across the field. Trials in heterosexual serodifferent couples, such as Partners PrEP, reported higher efficacy where measured adherence was higher, while studies in populations that struggled with daily dosing produced lower or even null results driven by low drug levels rather than a failed compound. The Bangkok Tenofovir Study in people who inject drugs is the basis for the CDC's at-least-74-percent figure for that route, and its protection also climbed with adherence. This is the core lesson: a PrEP efficacy percentage is not a fixed property of a molecule but a joint product of the drug and how consistently it reaches protective levels.

How you measure a prevention drug once placebo becomes unethical

Once daily oral PrEP was proven, withholding it in a placebo group was no longer ethical, which changed how newer agents could be evaluated. Instead of placebo, later trials leaned on two reference points: an active control, meaning an already approved oral regimen, and an estimated background incidence, the rate of new infections expected in a comparable untreated population, often reconstructed using recency assays that distinguish recent from long-standing infection. That design is what allowed the long-acting studies to demonstrate superiority rather than only non-inferiority.

Long-acting injectables remove the daily variable

The most direct test of the adherence hypothesis came from long-acting cabotegravir, given by injection every two months. In HPTN 083, cabotegravir proved superior to daily oral emtricitabine-tenofovir among men who have sex with men and transgender women, with roughly one-third the infections seen in the oral group. A companion trial, HPTN 084, showed a similarly large advantage in cisgender women. The injection did not carry a categorically stronger drug; it guaranteed drug levels that the daily pill frequently failed to reach in ordinary use. The gap between what the pill could do and what it did in practice was, in large part, the gap that the injectable closed.

Twice-yearly lenacapavir and how the evidence was graded

The same logic scaled to a capsid inhibitor given under the skin twice a year. In PURPOSE 1, conducted among adolescent girls and young women in South Africa and Uganda and published in the New England Journal of Medicine, there were zero HIV infections among the 2,134 participants who received lenacapavir, an incidence significantly below both the reconstructed background rate and the oral comparator. In PURPOSE 2, among men and gender-diverse people, two infections occurred among 2,179 participants, meaning about 99.9 percent remained uninfected and representing roughly a 96 percent reduction against background incidence. The FDA approved twice-yearly lenacapavir in June 2025 as the first PrEP option dosed every six months, and in September 2025 the CDC recommended it for people who would benefit from PrEP, citing a high certainty of evidence for its efficacy and safety.

That grading language is not decoration. Under structured frameworks such as GRADE, the certainty of evidence rises when the design is a randomized trial with a hard, objective endpoint (documented HIV seroconversion), when enrollment is large, when the direction of effect is consistent across studies, and when the effect estimates are precise. The PURPOSE program met those conditions, which is how a recommendation reaches high certainty rather than moderate or low. The strength of a recommendation reflects the quality of the evidence behind it, not merely the size of a percentage.

Reading the number honestly

The near-99 percent belongs to a person for whom the drug is reliably present. For a daily pill, that presence depends on daily behavior. For a bimonthly or twice-yearly injection, adherence becomes a scheduling and access question resolved at a clinic visit rather than every morning, which is why the newer options narrow the gap between what a regimen can do in a trial and what it does in everyday use. It also explains why a single figure on a fact sheet can be true and incomplete at the same time: the percentage is a ceiling that a given regimen approaches only to the degree the drug is actually onboard. Understanding that conditionality is what turns a marketing-friendly number back into a scientific claim.

This article is educational and not medical advice.