What is PrEP?
Pre-exposure prophylaxis (PrEP) refers to an experimental HIV-prevention strategy that would use anti-retrovirals (ARVs) to protect HIV-negative people from HIV infection. In this strategy, people would take the medications before they were exposed to HIV, in hopes that it would lower the risk of infection. PrEP is different from post-exposure prophylaxis (PEP), in which people who have been exposed through a needle, rape, etc., immediately begin taking ARVs in hopes of preventing infection.
What is the rationale for PrEP research?
Studies of PrEP strategies in non-human primates have shown a reduced risk of infection among animals that receive ARVs prior to exposure to a simian form of HIV. These studies compared small groups of animals pre-treated with ARVs, with animals that did not receive any ARVs and were exposed to the same virus. In these studies, there was evidence of protection from PrEP.
Another rationale comes from strategies to prevent mother-to-child transmission, which use ARVs given to the mother and the infant to help reduce the risk of transmission.
How is PrEP efficacy being tested?
All HIV-prevention trials use similar trial designs, although the exact protocol depends on the strategy being studied. Briefly, HIV-prevention trials are conducted in communities where the rate of new HIV infections—the “incidence”—is well-documented prior to the study launch. When the trial begins, volunteers are randomly assigned to either the experimental arm, which receives the strategy being studied, or to the control arm. Both arms receive the same package of prevention counseling and services, which includes provision of condoms, treatment of sexually-transmitted infections, and behavior change counseling in heterosexual transmission studies. Clean needles and harm-reduction services should be provided in injection-drug user studies in order to meet basic ethical requirements. Participants are never encouraged to have unsafe sex or to expose themselves to HIV.
After enrollment, all volunteers are followed over time, with periodic study visits where HIV tests and other lab analyses may be conducted. The data are periodically reviewed by a Data and Safety Monitoring Board, an independent body. At the end of the study, researchers look at the incidence rates in the experimental and control arms to see if there is a difference. If the strategy is effective, incidence should be significantly lower in the experimental arm.
In the case of PrEP trials, neither the study volunteers nor the staff know who is receiving the active drug and who is receiving the placebo – in this case, pills that look identical to the ARVs but have no effect. Volunteers are closely monitored for side effects associated with the medications.
For more information on how HIV prevention trials are conducted, click here.
For more information on PrEP trials, visit the PrEP Clinical Trials page.
For more information on how PrEP research fits into the “big picture” of HIV prevention research and also to learn more about other technologies in the research pipeline, visit the comprehensive timeline of HIV prevention research.
For additional background information on PrEP, click here for “Will a Pill a Day Prevent HIV?”
For additional introductory reading on “the basics” of PrEP research, check out the list of resources (left-hand column).
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