Some researchers conducting clinical trials on a COVID-19 vaccine have not revealed to the public what the placebo contains, but they should.
This is because the placebo ingredients influence how effective or harmful the active treatment, with which the placebo is compared, appears. Our new guideline published in
PLOS Medicine remedies this problem by providing a template for reporting what's in placebo controls.
In some COVID-19 vaccine trials, participants in the control group (the group receiving a placebo) are injected with a saline solution. In other trials, they receive an actual treatment.
For example, in the COVID-19 vaccine developed by the University of Oxford, the control group receives a meningitis and septicaemia vaccine as a placebo.
The benefit of using an actual vaccine as the placebo control is that it will cause a similar reaction at the site of the injection as the COVID-19 vaccine, such as muscle pain and soreness. This prevents patients from knowing whether they are getting the placebo or the real treatment. The scientific term for hiding knowledge of who got what treatment is "blinding".
If patients know they are getting the real thing, they may expect to get better, and their expectations can make them
get better a bit faster. And if they know they are getting the placebo, they could
drop out of the trial because they know they aren't getting the actual treatment. Adding an actual vaccine to the placebo control helps the trial remain blinded and so prevents bias arising from differing expectations.
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