It’s still not fully understood how placebos work – but an alternative theory of consciousness could hold some clues
Steve Taylor does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.
If you’ve had both of your COVID vaccinations, you may have suffered some side-effects – perhaps headaches, fatigue, fever or a sore arm. These effects are mainly caused by your immune system’s reaction to the vaccine. But most scientists agree that there is another cause: the human mind.
The ability of the mind to generate the symptoms of illness is known as the “nocebo” effect. The nocebo effect is the unpopular twin brother of the placebo effect. Whereas the placebo effect alleviates pain and the symptoms of illness, the nocebo effect does the opposite: it generates pain and symptoms.
A 2018 study found that almost half of participants in placebo trials experience side-effects, even though they are taking inert substances. There was a similar finding in the first major trial of the Pfizer COVID vaccine in 2020. In the placebo group – who were not given the vaccine – between a quarter and a third of people reported fatigue, a similar number reported headaches, and around 10% reported muscle pain.
