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.
Phase
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.