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Harvard epidemiologists have calculated that about 76% of systemic adverse reactions after the first dose of coronavirus vaccines can be explained by the nocebo effect.
This effect, the expectation of negative consequences, can also explain 52% of systemic reactions after the second dose.
To find out, the researchers performed a meta-analysis of 12 studies that included a total of 45,380 participants.
A placebo is a substance with no real medicinal properties, which is used in clinical trials in order to evaluate the effect of a real drug.
The placebo effect can only occur if the person believes in the effectiveness of the dummy substance.
The Nocebo is the antithesis of placebo, also a substance without real pharmacological properties.
However, unlike placebo, it causes a negative reaction in the patient, because people expect a negative effect from the drug.
The nocebo effect can often be seen in clinical trials when patients report side effects after taking a dummy drug.
Clinical trials of coronavirus vaccines were no exception.
In order to figure out how common the nocebo effect is among their participants, a group of researchers led by Ted J. Kaptchuk from Harvard Medical School decided to study the phenomenon.
They performed a meta-analysis of 12 studies with a total of 45,380 participants.
All studies examined the effectiveness of coronavirus vaccines in two groups of participants: one received a real drug, and the other received a dummy.
After the first dose in the placebo group, 35% of participants reported systemic adverse reactions (headache in 19% and fatigue in 17%).
Among the people who received the vaccine, there were more such participants - about 46%.
After the second dose, 32% of people in the dummy group and 61% of the drug group reported side effects.
The difference in the number of side effects between the experimental and control groups was statistically significant.
However, the scientists calculated that the nocebo effect was responsible for 76% of systemic adverse reactions after the first dose and 52% after the second dose.
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