New research suggests that a high percentage of reported side effects from COVID-19 vaccines may be due to what’s known as the ‘nocebo’ effect. The ‘nocebo’ effect is essentially the opposite off the placebo effect – a biopsychological phenomenon that occurs in many patients as a product of their response to a placebo. In the case of the ‘nocebo’ effect, this produces an adverse event as opposed to a beneficial event. 

“The high number of nocebo responses found in this study [is] not at all surprising,” says Cognivia CBO Erica Smith, PhD, in this Medical News Today article. “It is well-known that expectation contributes significantly to both placebo and nocebo responses in clinical trials.”

The study found that a third of people who received placebo during COVID-19 vaccine clinical trials reported unpleasant systemic side effects. Subsequently, the authors of that study find that 76% of side effects reported by actual vaccine recipients are likely attributable to nocebo responses. 

“The findings of this study — that a significant proportion of adverse events reported in clinical trials for COVID-19 vaccines are unrelated to the treatment itself and are instead attributable to the nocebo effect — can and should be leveraged by the medical community with the hopes of increasing vaccine acceptance,” says Smith.Learn more about the study, nocebo response and adverse event expectations in the full article.