The scientists examined data from 12 clinical trials of Covid vaccines.
They found the ‘nocebo effect’ accounted for about 76% of all common reactions after the first dose and nearly 52% after the second.
Here’s what we know so far...
What is the ‘nocebo effect’?
A nocebo effect refers to symptoms related to a person’s negative expectations.
The findings from the study suggest that a substantial proportion of milder side-effects, such as headaches, short-term fatigue, and arm pain are not produced by the vaccine.
Instead, they are caused by other factors generating the nocebo response such as anxiety and negative expectations of the jab.
What have researchers said about the ‘nocebo effect’?
The researchers argue that better public information is needed about nocebo responses as it will reduce concerns that are making people hesitant which will in turn improve the Covid vaccine uptake.
Ted Kaptchuk, professor of global health and social medicine at Harvard medical school, and a senior author on the study, said: “Telling patients that the intervention they are taking has side-effects that are similar to placebo treatments for the condition in randomised controlled trials actually reduces anxiety and makes patients take a moment to consider the side-effect.
“But we need more research.”
He added: “Most researchers argue that patients should be told less about side-effects to reduce their anxiety. I think this is wrong. Honesty is the way to go.”
How was the study carried out?
Kaptchuck and Dr Julia Haas at the Beth Israel Deaconess Medical Center in Boston analysed adverse effects reported during 12 clinical trials of Covid vaccines.
In every trial, those in the placebo arm were given injections of inactive salt solution instead of the vaccine.
The study did not account for severe, rare side-effects such as blood clots or heart inflammation.
What was found?
Writing in the journal Jama Network Open, the researchers described the findings.
After the first injection more than 35% of those in the placebo groups experienced “systemic” side-effects such as headache and fatigue.
No16% reported site-specific effects such as arm pain, redness, or swelling at the area of injection.
Those who received a first shot of vaccine were more likely to experience side-effects. Around 46% reported systemic symptoms and two-thirds experienced localised symptoms at the area of injection.
Looking at the side-effects after the second jab, researchers found the rate of systemic symptoms were nearly twice as high in the vaccine group compared to the placebo group - at 61% and 32%.
The difference was greater for localised symptoms, reaching 73% in those who had the vaccine and 12% in the placebo group.
Overall, the researchers calculate that around two-thirds of common side-effects such as headaches and fatigue reported in Covid vaccine trials are driven by the nocebo effect.
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