IMHO, drawing conclusions about the safety and efficacy of a new vaccine by giving an approved vaccine (that could very well have potential side effects, per its package insert) to both sides of the clinical study is a blatant distortion of the scientific method.

Placebo and drug kits in clinical trial design
WHO Drug Information Vol. 31, No. 2, 2017
https://www.who.int/medicines/publications/druginformation/issues/WHO_DI_31-2_Placebo.pdf

Introduction
Before novel medicinal products are introduced into widespread use, they must be assessed in clinical trials. Randomized controlled trials (RCTs) are often considered the gold standard in this regard, although other study designs can also yield valid research results. Clinical trials should be designed in such a way that the effects of the experimental intervention are compared with those of a control intervention. In a controlled trial, the subjects in the study and control group should be drawn from the same population, and should preferably be assigned to the groups by randomization to remove bias in the allocation of participants.

Where feasible, clinical trials should be blinded, so that the subjects – and in double-blinded studies also the researchers – are unaware of who is receiving which intervention. This helps to avoid behaviour changes that may influence the study outcomes.

The use of placebo
A placebo has been defined as “an inert substance or sham procedure that is provided to research participants with the aim of making it impossible for them, and usually the researchers themselves, to know who is receiving an active or inactive intervention.”

Placebos typically consist of the ingredients employed in the medicinal product under study minus the active ingredient, making them inert. The inactive ingredients (excipients) employed in a pharmaceutical product must be “generally recognized as safe”1 for use in humans, otherwise a medicinal product would not be authorized for use.

In vaccine research, the term “placebo” is also applied to non-inert substances. In this context, an existing vaccine not studied in the trial is added to both the investigational and the control product in order to avoid giving an “empty” injection to the subjects in the control group. A disadvantage of this approach is that it complicates the evaluation of the safety and reactogenicity of the vaccine under study