The vaccine, which is showing good results in animal experiments, is further tested in three phases before its large-scale use.

• Phase I vaccine trial: It is the first human tests of a candidate vaccine, generally conducted on small numbers (10-30) of healthy adult volunteers who are not at risk for the disease. The main goal is evaluation of safety, and to a lesser extent, analysis of the immune responses evoked by the vaccine. A Phase I trial usually takes 8-12 months to complete.
• Phase II vaccine trial: It involves a larger number of volunteers (50-500), usually a mixture of low-risk people and higher-risk individuals from the population where Phase III (vaccine efficacy) trials will eventually be conducted. Phase II trials generate additional safety data as well as information for refining the dosage and immunization schedule. Although not set up to determine whether the vaccine actually works, Phase II trials are sometimes large enough to yield preliminary indications of efficacy. These trials generally take 18-24 months.
• Phase III vaccine trial: Phase III trials are the definitive test of whether a vaccine is effective in preventing disease. Using thousands of volunteers from high-risk populations in geographic regions where HIV is circulating, the incidence of HIV in vaccinated people is compared to that in people who receive a placebo. Successful demonstration of efficacy in a Phase III trial can then lead to an application for licensure of the vaccine. Phase III trials of AIDS vaccines are generally expected to require a minimum of three years for enrollment, immunizations, and assessments of efficacy.

Currently there is only one vaccine in phase III. Two are in phase II and more than twenty are in phase I.

First phase I trial will be conducted atin India in year.

The scientific consensus is that an AIDS vaccine is possible. This is because

• Experimental AIDS vaccines have protected non-human primates (monkeys).
• It has been found that there are rare individuals who exhibit an exceptional ability to tame the virus. Analysis of what is different about immune system of these individuals is yielding idea for vaccine development. For example, some female sex workers have remained HIV uninfected for years, despite repeated exposure to HIV. Researchers are building and testing vaccines designed to induce similar immune response as shown by these women.
• Vaccines have controlled other infectious diseases such as small pox, Polio and Measles.
• Advances in molecular biology and basic HIV research have led to the development of promising strategies for effective AIDS vaccines.

The major obstacle in HIV vaccine preparation is that, the virus continuously changes its surface makeup and evolves to escape defending mechanism against it.
There are nine different subtypes of HIV across the globe and there is large variation within each subtype making vaccine preparation a difficult task.
HIV infects immune cells needed to fight disease and also develops reservoirs of virus by integrating into the host DNA.

Recently, there have been promising breakthroughs in research on vaccines that reduce viral load, thereby decreasing chances of transmission from someone who is HIV positive to someone who is not. A preventative HIV vaccine, however, may still be years away.

Priming is to give one type of vaccine dose to induce certain kind of immune response, to be followed by or together with a second type of vaccine (booster). A prime-boost combination may induce different types of immune responses and/or enhance overall responses beyond those seen with only one type of vaccine.

In randomized trial participants are assigned by chance to experimental or control arm of the trial. Randomization minimizes the differences among groups by equally distributing people with particular characteristics among all the trial arms.
In blinded vaccine trial participants are unaware whether they are in the experimental or control arm of study. A clinical trial in which neither the study staff nor the participants know which participants are receiving the experimental vaccine and which are receiving placebo is called double-blind study.