A clinical study involves either observational research or research that studies medical, surgical, or behavioral interventions in people, often called a clinical trial. They are the primary way researchers conclude if a new treatment, like a new drug, is safe and effective in people.

In the HPTN’s HIV prevention clinical studies, we examine use of antiretroviral drugs (antiretroviral therapy and pre-exposure prophylaxis); interventions for substance abuse, particularly injection drug use; behavioral risk reduction interventions, and structural interventions.

There are four phases of clinical studies.

pillPhase 1 Clinical Studies

Phase I studies test the safety of a product. This means seeing if there are side effects, and whether the human body can tolerate the experimental product, such as a new drug. This product is often compared to a group that receives a placebo, a substance such as sterile saltwater that has no active ingredients. These studies are conducted with a small group of people (usually less than 100) and typically last 12 to 18 months.


beakerPhase 2 Clinical Studies

Phase 2 studies look at questions such as the maximum tolerated dose of a product, the optimal schedule for giving the product (how many doses, and at what time intervals), and whether the immune system is having the desired responses. These responses can include the production of antibodies, the production of T cells, and other immune system markers. These studies are conducted with a medium-size population of volunteers (usually a few hundred to 1,000) and the studies can last up to two years.


ClipboardPhase 2b Clinical Studies

Phase 2b studies are a way to get an early look at whether the product is effective at preventing infection or disease. They are sometimes known as Proof of Concept or Test of Concept studies. A group of several thousand people who are at increased risk for infection or disease are enrolled, and the studies can last about 2-5 years. Based on the data from these studies, researchers can see whether the results seem favorable, supporting moving ahead to Phase 3. If the results are less favorable, it means that researchers can redirect their efforts and save the expenses of large Phase 3 studies.


microscopePhase 3 Clinical Studies

Phase 3 studies are where researchers can ask the questions, “Does this product prevent new infections?” Or “if people do become infected, does the product help them control the infection so that it doesn’t become severe disease?” These studies involve many thousands of people, usually including some participants who are at increased risk for infection.



bookPhase 4 Clinical Studies

Phase 4 studies may take place after a product has been found to be effective in some populations and are utilized to gather additional information about safety and effectiveness by testing the product in additional populations such as children or pregnant women. They are sometimes known as bridging studies, or post-licensure studies. These studies may be conducted in anyone seeking a prevention option from their physician.