In research of any kind it is vital to have both an experimental group and a control group.
The experimental group receive the treatment the researcher is interested in measuring the effectiveness of. The control group does not receive the treatment.
The reason for this is so that the researchers can determine if the treatment has an impact on the measure of experiment (for example, a reduction in serious infection as well as the incidence of adverse reactions in those given the treatment).
A control group serves as a baseline comparison to the experimental group, whereby the researchers can reliably compare the measured results of the treatment cohort with the control.
Failure to provide a strong control group may cause a study to be considered invalid, because it would not allow the researchers to eliminate or attribute effects that might have occurred within the experimental group as being due to the treatments or incidental occurrence.