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Vaccine Development  Phases (Stages) :
Vaccines that protect against infection are typically developed through a series of different studies, or trials. After basic biological research and animal studies have been completed, progressive clinical trials in humans are conducted. Advancement from one phase of trials to the next depends on the successful completion of the previous set of trials. 
ase I
The first setting for vaccine evaluation in humans is a Phase I trial in which the vaccine is tested in a small number (20-80) of healthy, low-risk, uninfected volunteers to determine the safety of the candidate vaccine and the optimal dosage and immunization schedule. 

 Phase II
If the vaccine successfully produces the desired immune response and it is well tolerated, a Phase II trial is conducted in larger numbers (up to a few hundred) of healthy, uninfected volunteers to further establish safety and to refine the dosage and immunization schedules. 

 Phase III
Promising vaccine candidates then advance to a phase III trial, which is a much larger-scale trial involving thousands of uninfected, high-risk individuals to determine the protective efficacy of the vaccine. This is the last and most important step in the evaluation process before a vaccine is considered for licensing. Phase III efficacy trials usually require the use of a placebo, an inactive substance given to some individuals to compare the effect of the vaccine.

   Over the last decade, 34 different preventive HIV candidate vaccines have entered phase I clinical trials, and 3 have entered phase II trials. The candidate vaccines developed and tested to date have been found to be safe and well tolerated, and nearly all have produced HIV-specific immune responses with varying degrees of success. The knowledge gained in these trials led to the first phase III vaccine trial, which began in the United States in June 1998. Thailand will become the first developing country to implement a phase III trial in January 1999.

   It is anticipated that multiple phase III efficacy trials will be necessary in both developed and developing countries. Similar to vaccines for other diseases developed thus far, the first HIV vaccine(s) may have limited protective efficacy. As with many other vaccines previously developed, the early HIV vaccines will hopefully quickly lead to second-generation vaccines with improved efficacy or safety.  It is important to note that efficacy "failure" can occur, in that some candidate vaccines may not provide sufficient, or indeed any, protection. Such outcomes are sometimes part of the development process leading to an effective vaccine. Researchers must prepare the community, government, and the medical and public health community for the possibility of such outcomes. Nonetheless, such trials can be an important step forward if well designed and conducted to allow clear conclusions, if the population is well informed as to the potential risks and benefits, and if important information is learned that will advance vaccine development.