The experimental group in any controlled science experiment is the group exposed to the variable being tested. This is different than the control group, which is not exposed to the variable. In an experiment for a vaccine, the experimental group receives the vaccine, and the control group does not.
All controlled science experiments require both an experimental group and a control group. This way the results of the variable being introduced to the experimental group can be compared with the results of no introduced variable in the control group. The differences in results can then be measured to deduce any effects of the variable upon the subjects of the experiment.