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Logistical issues in Phase III trials
The size and length of follow-up for a Phase III (efficacy) trial will depend on:
- the incidence of HIV in the population where the trial is taking place
- the likely efficacy of the vaccine
- volunteer drop-out and non-response rates
- the trial design, and the outcome(s) being assessed (as previously discussed).
If a trial is too small, runs for too short a period, and the vaccine has a limited effect, then it may be inconclusive. Alternatively, if a trial is too big, it could waste resources.
Once a vaccine has been identified which is at least partially effective, it would become necessary to compare that vaccine with other potential vaccines. These trials might be looking for simpler vaccination schedules giving at least the same level of protection, longer duration of protection, and/or reductions in new HIV cases from a lower baseline. All of this implies that trial size and/or duration needs to increase as soon as a partially effective vaccine is identified.