Headaches, fatigue, body aches… Many people say they experience these side effects after being vaccinated against Covid-19. However, the majority of these adverse effects could be attributable to the nocebo effect and not to the content of the vaccine itself, according to a study published on Monday January 18 by American researchers. [étude en anglais].
The nocebo effect is the negative version of the placebo effect. In the same way that some people can feel relief just by ingesting a tablet, even if it only contains sugar for example, other people systematically experience certain side effects listed on the leaflet or that they have heard about. , whether the treatment is real or does not contain any active substance.
The American researchers therefore wanted to quantify this nocebo effect after vaccination against Covid-19. These scientists from a Boston hospital, in connection with Harvard University, were interested in the reports of adverse effects of 45,000 volunteers involved in twelve clinical trials of vaccination against Covid-19, all vaccines combined. To check the effectiveness of the injection, half of the volunteers received a real vaccine and the other half a placebo.
The results show that those who only received a neutral injection still experienced side effects in the days following the injection. After analyzing all the data, these researchers conclude that 76% of the adverse effects occurring after the first dose of the real vaccine against Covid-19 are due to this nocebo effect. This is also the case for 52% of reactions after the second dose.
According to these researchers, the majority of side effects are therefore not related to the content of the vaccine against Covid-19. More than half of these side effects occur because you expect to feel them. It is mainly fatigue and headaches. But these undesirable effects are not imaginary for the patient; they are very real.
The researchers’ hypothesis is that it is stress or anxiety that induces this nocebo effect. This is also perhaps why it is less important during the second injection: these researchers explain it by greater patient confidence. Hence the importance, they conclude, of the transparency of information – including on this nocebo effect – and of the quality of the relationship with one’s doctor.