Phases of Testing and Clinical Trials

It all starts with a bright idea! Researchers working in academia, pharma, biotechs and research centers are constantly testing out their theories.

The most promising ideas advance to the laboratory, where things can be tested in tissue cultures or tried out in mathematical models and computer simulations. Ideas are refined, and from the best approaches an experimental vaccine is developed.

Pre-Clinical Trials

Animal trials are also known as pre-clinical trials. These often include mice, rabbits and rhesus macaques. But no matter how much we learn and study in animal models, only people become infected with HUMAN immunodeficiency virus. Since no animals have the same immune systems as humans, the only way to prove a vaccine's effectiveness is to test in people, so ultimately we have to move forward into human trials. Very few vaccines reach this point.

Animals in Pre-Clinical Trials

Animals in Pre-Clinical Trials

Phase I Trials

Phase I involves a small number of healthy, uninfected participants at low risk of HIV infection, and primarily tests the safety of the preventive vaccine. This is done by comparing the vaccine with a control or placebo (an inactive substance, such as normal saline). A Phase I trial can also provide initial data on the dose and administration schedule (the time between vaccinations) that achieve the optimal immune response. Phase I trials usually last 12-18 months.
20-100 volunteers in Phase I Trial

20-100 in Phase I
Phase II Trials

Phase II involves hundreds of participants with varying degrees of risk to better characterize the safety of the vaccine and the immune response being caused by the vaccine. Questions about the right dose and the scheduling of injections can usually be further sorted out here. Phase II trials can last 2 or more years.
Hundreds of volunteers in Phase II trials

Hundreds in Phase II

Phase III Trials

After a successful Phase II trial, a Phase III trial involves several thousand high-risk volunteers to further assess if the vaccine works in preventing HIV infection. Phase III trials can last 3-5 years.
Thousands of volunteers in Phase III trials

Thousands in Phase III


Volunteers in clinical trials cannot get HIV infection or AIDS by receiving an experimental vaccine.

An experimental vaccine must successfully complete at least three stages of testing in people before it can be licensed. Human clinical trials are regulated by strict ethical and scientific controls, and occur at specialized research centers around the world.