If the vaccine triggers the expected immune response;
If the vaccine is safe to move into larger studies;
Which doses can be adequate.
Therapeutic exploratory studies (phase II trials) involve several hundred volunteers. The purpose of this phase is to study the best doses to use, the most common side effects and how many doses are needed.
These studies also check that the vaccine triggers a good immune response in a broader population.
In certain cases, it could also provide some preliminary indications of how well the vaccine will work (efficacy).
Clinical efficacy and safety studies (phase III trials) include thousands of volunteers.
This phase shows how efficacious the vaccine is at protecting against the infection compared with placebo (dummy) or alternative treatment and what are the less common side effects in those receiving the investigational vaccine.
The reduction in number of people with symptoms, severe disease or diagnosed with the infection can help to measure the efficacy of the vaccine.