If the placebo effect describes the benefits felt by a patient who has been administered an inert substance, the nocebo effect rather describes the harmful effects.
Researchers at Beth Israel Deaconess Medical Center, affiliated with Harvard University, studied data generated from placebo-controlled clinical studies of COVID-19 vaccines.
While they found significantly more side effects in participants who did receive a vaccine, about a third of subjects who received a placebo also complained of side effects, with fatigue and headaches occurring at the top of the list.
The meta-analysis looked at 12 studies involving some 45,000 people split evenly between a vaccine and a placebo.
After the first injection, more than 35% of the subjects who had received a placebo experienced a systemic side effect, and 16% a localized side effect. By comparison, 46% of participants who received a vaccine reported a systemic side effect, and two-thirds (67%) a localized side effect.
Since many of these effects also occurred in the placebo group, the authors conclude that 76% of side effects in the vaccinated group were attributable to the nocebo effect.
After the second dose, the placebo group reported fewer side effects than after the first (32% systemic and 12% localized), but the vaccinated group reported more (61% systemic and 73% localized).
The researchers attribute 52% of the side effects experienced after the second injection to the nocebo effect. They hypothesize that it could be a combined effect of an increased immune response and the fact that vaccinated people expected more effects after their second dose.
Researchers believe that some people mistakenly attribute mundane everyday problems to the vaccine, in part because they have been amply informed about the potential side effects of vaccination. These warnings could also cause stress and anxiety, they say, and make people hypervigilant to the slightest sensation.
The authors acknowledge that their analysis covers a relatively small number of studies, with a variety of approaches, and that caution should therefore be exercised in interpreting the results.
The researchers nevertheless believe that it would be advantageous to inform the population of this nocebo effect since this could, according to them, counter the reluctance of some to be vaccinated.
The findings of this study are published by the medical journal JAMA Network OpenHave (New window)Have.