Daily oral PrEP using TDF/FTC provides high levels of protection against HIV in people who take the pill regularly. But this protection doesn’t happen overnight. Instead, a person needs to take a number of doses to build up protective levels of the drug in the blood.
Just how many doses?
Right now, the answer to this question is an educated guess—and the World Health Organization (WHO) and the US Centers for Disease Control and Prevention (CDC) have different answers about “time to protection” in their respective guidelines for oral PrEP use.
Please join us for a webinar on the data behind “time to protection for PrEP” on Thursday, February 9, 11am–12:30pm US Eastern Time (visit www.timeanddate.com for the local time in your area) to learn more. This webinar will include pharmacologists who have studied drug levels in the blood and tissue of PrEP users, as well as representatives from the WHO who were involved in developing the guidance on this topic along with advocates and implementers.
The primary difference between US CDC and WHO guidelines on time to protection relates to women. Specifically, US CDC guidelines recommend that women complete 20 doses of daily oral TDF/FTC to achieve protective levels of the drug in the vaginal tissue. WHO recommends seven days for men (penile and rectal exposure) and women (vaginal and rectal exposure).
Both of these recommendations are based on measurements of the amount of drug that accumulates in blood and/or tissue over a specific period of time. The studies of how drugs are taken into the body and how they leave the body is called “pharmacokinetics” and “pharmacodynamics” or “PK” and “PD” for short, as explained in our primer for advocates (www.avac.org/pharmacokinetics-and-pharmacodynamics). There isn’t a single PK measurement that is associated with PrEP protection—so both WHO and CDC guidelines are based on inference.
When indirect measures are used for direct conclusions, advocates need to understand the rationale. We hope this webinar will further the conversation. Please join us.