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Pre-exposure prophylaxis (PrEP) is any medical or public health procedure used before exposure to the disease causing agent, its purpose is to prevent, rather than treat or cure a disease. An example would be if a doctor gave a medication used to treat a disease to a healthy person who is not thought to have that disease, but is at risk for contracting it. More specifically, this practice is common with people who are about to travel from an area without malaria to an area where malaria is a risk. In the U.S., it is approved by the FDA and recommended by the CDC as a tool to prevent persons from contracting HIV.
Pre-exposure prophylaxis can also refer to the aggressive use of vaccination, for example in an attempt to prevent rabies in people such as laboratory workers who are high risk for being bitten by rabid animals.
Some of the factors in deciding whether to use chemotherapy as malaria PrEP include the specific itinerary, length of trip, cost of drug, previous adverse reactions to antimalarials, drug allergies, and current medical history.
The terms pre-exposure prophylaxis or PrEP most commonly refer to an HIV-prevention strategy that uses antiretrovirals to protect HIV-negative people from HIV infection. The HIV antiviral Truvada was approved by the FDA for PrEP on July 16, 2012. The CDC added Truvada for PrEP to its guidelines for HIV prevention on May 14, 2014.
Another rationale of PrEP 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.
Most PrEP studies utilize the drug tenofovir or a tenofovir/emtricitabine combo (Truvada) that is delivered orally. Initial studies of PrEP strategies in non-human primates showed a reduced risk of infection among animals that receive ARVs prior to exposure to a simian form of HIV. A 2007 study at UT-Southwestern (Dallas) and the University of Minnesota showed PrEP to be effective in "humanized" laboratory mice. In 2008, the iPrEx study demonstrated 42% reduction of HIV infection among men who have sex with men, and subsequent study of the data indicated 99% protection with daily adherence. 
PrEP approaches with agents besides oral Truvada are currently in clinical trials not listed here.
|Study||Type of PrEP||Study Population||Findings|
|CAPRISA 004||Pericoital tenofovir gel||South African females||39% reduction of HIV infection|
|iPrEx||Oral emtricitabine/tenofovir||Men who have sex with men||42% reduction of HIV infection. 99% reduction estimated with daily adherence|
|Partners PrEP||Oral emtricitabine/tenofovir; oral tenofovir||African heterosexual couples||73% and 62% reduction of infection|
|TDF2||Oral emtricitabine/tenofovir||Botswana heterosexual couples||63% reduction of infection|
|FEM-PrEP||Oral emtricitabine/tenofovir||African females||No reduction (study halted due to low adherence)|
|VOICE 003||Oral emtricitabine/tenofovir; oral tenofovir; vaginal tenofovir gel||African females||No reduction in oral tenofovir or vaginal gel arms [oral emtricitabine/tenofovir arm ongoing]|
|Bangkok Tenofovir Study||Oral tenofovir||Thai male injection drug users||48,9% reduction of infection|
Criticisms of PrEP have included biomedical concerns (such as safety and effectiveness), adherence, behavioral repercussions, and cost.
The PrEP studies have shown the drugs to be safe, with few side effects. Generally, minor side effects such as nausea or diarrhea resolve themselves within the first few months. Effects of Truvada on kidney function have been shown to be temporary. The level of effectiveness depends on the degree of adherence to the prescribed regimen. In iPrEx, persons with greater than 90% adherence to the drug had an efficacy of 68% protection versus persons with less than 50% adherence who had an efficacy of 16%. Furthermore, the FEM-PrEP trial that was stopped early due to futility, found that the women's adherence to the drug was too low to find any effect on reducing HIV infections.
Given mounting evidence of the effectiveness of PrEP among different populations when the drug is taken correctly, concern has turned more towards the practicalities of implementing PrEP as prevention. Since approximately 60% of people needing anti-retroviral therapy are not getting it, there are concerns that trying to deliver PrEP to many more people would be challenging. It is also important to consider how programs might target those at highest risk of HIV exposure in order to provide PrEP.
Effects of PrEP on behavioral changes, such as decreased condom use, are currently being studied. The CDC is conducting an extended safety trial in the U.S. to determine if men who have sex with men taking the drug are more likely to engage in riskier behaviors; preliminary results do not show any change in behavior.
Cost has been cited as a concern as well. Truvada for PrEP is listed at more than $1000/month. While many insurers cover the treatment, and while Truvada's manufacturer, Gilead, maintains a co-pay assistance program, the treatment may be out of the reach of some persons. The effects of PrEP on overall health care costs are uncertain.