After many years of testing in the lab and on animals, all drugs, including vaccines, go through three phases of clinical trials in humans in order to be licensed by the Food and Drug Administration (FDA) for public use.

• Phase I trials are conducted to test for safety. The vaccine is tested in a small number -- a few dozen or so -- of healthy individuals who are at low risk of infection. Phase I trials, which typically last 12 to 18 months, may also test various dosages of the vaccine. The individuals are continually monitored to see if there are any side effects, and their blood is tested for immune responses.
• Phase II trials continue to test safety, side effects and immune responses, but this time in several hundred healthy individuals, including some who may be at high risk for becoming infected with HIV. These trials can last up to two years.
• Phase III trials, known as "efficacy" trials, test for safety and to see if the vaccine actually works. These trials involve several thousand healthy volunteers and can last up to three or four years. They are usually "double-blind" placebo-controlled studies, meaning neither the patients nor the clinicians know which participants have received the vaccine and which have received a placebo, an inert substance that looks like the drug.

Potential AIDS vaccines may also undergo a few additional trials:

• Phase IIb trials, sometimes called "proof of concept" trials, test the vaccine for efficacy in a smaller number of people; for example, 2,000 to 5,000 volunteers, as compared to the 10,000 individuals who would enlist in a Phase III trial. The idea is that Phase IIb trials are a cost-effective way to look for preliminary evidence as to whether the vaccine will work before moving to the more expensive Phase III trials. [Read more on the advantages of a Phase IIb trial from the International AIDS Vaccine Initiative.]
• Phase IV trials, which involve many thousands of people, are conducted after a vaccine is licensed as a way to continually measure efficacy; for example, whether immunity lasts or wears off after a certain amount of time.

According to the AIDS Vaccine Advocacy Coalition (AVAC), while several dozen vaccines have made it to Phase I trials, only three experimental HIV vaccines have been tested in Phase III trials.