Researchers have found that more than two thirds of the common side-effects reported after a Covid vaccine dose can be attributed to the "nocebo effect", a negative version of the placebo effect, rather than the vaccine itself.
A study that looked at data from 12 clinical trials for Covid vaccines found that the "nocebo effect" was responsible for 76 per cent of the common adverse reactions reported after the first dose, and nearly 52 per cent of reactions after the second dose.
According to the results of the study, a large percentage of the milder side-effects experienced by people are unrelated to the constituents of the vaccine. Symptoms such as short-term fatigue, headaches and arm pain were found to be far more likely to be generated by the nocebo response, which can relate to anxiety surrounding the vaccine.
Researchers hope that educating the public about the nocebo responses may improve vaccine uptake by those likely to be hesitant.
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"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," said Ted Kaptchuk, professor of global health and social medicine at Harvard medical school. "But we need more research," he added.
Kaptchuck and Dr Julia Haas, from the Beth Israel Deaconess Medical Centre in Boston, US, looked at the data from the 12 clinical trials, which all included groups taking the inactive salt solution as a placebo, instead of the vaccine.
The study did not look at severe rare side effects such as blood clots.
Their findings were published in the Jama Network Open journal, where they described how more than 35 per cent of those in the placebo group, who had taken the inactive salt solution, experienced "systemic" side effects, which included headaches and short-term fatigue.
Looking at the side effects of the second jab, the researchers found that the rate of systemic symptoms, such as headaches, was nearly twice as high in the vaccine group, compared to the placebo group.
In total, the scientists estimate that around two-thirds of commonly reported side effects in the Covid vaccine trials could be attributed to the nocebo effect.