Some side effects of anti-Covid-19 vaccines are very real, others may be due to the nocebo effect, the negative counterpart of the placebo effect.
Even before they are available on the market, vaccines anti-Covid-19 inspired fear in many people, in particular because of the adverse effects they could cause. Families, friends and colleagues all discussed their condition in the days after the first injection, then the second. Some felt almost nothing, just arm pain, while others spent three days in bed, tired and stiff, even feverish. The fear of having a bad reaction to the vaccine is one of the main causes of refusal of vaccination by a minority of the population.
Yet, while everyone’s experiences are real, their connection to Covid-19 vaccines may not be so obvious. In the clinical tests, the group having received a placebo inert solution, saline for example, has also experienced adverse effects. This is called thewedding effect, the obscure counterpart of the placebo effect.
Placebo and nocebo
The placebo effect is well known to the general public and widely studied by scientists. It is when a neutral intervention causes positive effects on the patient’s health. A better mood, moreenergy, improved well-being. The reassuring words of a doctor can also have a placebo effect. Without treating anything, the placebo effect can still improve the general condition of the patient.
The nocebo effect is defined as: a harmless treatment, which when given, is associated with negative effects or aggravation of certain symptoms due to the patient’s negative expectations or psychological state. Both phenomena are largely psychological, but induce real symptoms: positive for the placebo effect, negative for the nocebo.
76% of post-vaccination side effects of psychosomatic origin
A meta-analysis published in Jama Network Open on January 18, 2022, analyzed the proportion of side effects attributable to the nocebo effect after the injection of anti-Covid-19 vaccines. The Harvard Medical School scientists included 12 randomized clinical trials, conducted on volunteers over the age of 16, which included a “vaccinated” group and an “inert placebo” group for which side-effect monitoring was available within seven days after injection.
The proportion of local and systemic side effects was calculated for the vaccinated and placebo group for this publication set. According to their results, 35.2% of people who received a primary injection of placebo reported a systemic side effect, most often fatigue and headaches; 31.8% after the second injection.
In comparison, 46.3% of people who received their first dose of the Covid-19 vaccine reported the same type of symptoms; and 61.4% after the second dose.
Scientists have thus determined that 76% of the systemic side effects that appear after the first injection of an anti-Covid-19 vaccine are attributable to the nocebo effect, and 51.8% after the second dose.
Very real local side effects
The nocebo effect makes vaccination more unpleasant than it actually is. Not all post-vaccination side effects are due to our apprehension or anxiety about to vaccines. The local reactions are indeed the result of the vaccine and its formula which activates the immune system.
Only 16.2% of volunteers in the placebo group reported a local adverse side effect after the first injection; 11.8% after the second injection. While 66.7% of volunteers in the vaccinated group experienced a local reaction after the first dose and 72.8% after the second. Thus, 24% of local side effects are attributable to the nocebo effect after the first injection, 16.2% after the second.
It’s not just a question of psychology. Nevertheless, discussing with the public the nocebo effect or the possibility that no unpleasant side effects appear could have a beneficial effect and limit the occurrence of non-specific symptoms, such as fatigue and headache. ” In addition, informing the public about a potential nocebo effect could help reduce concerns about Covid vaccines, which could also reduce vaccine hesitancy. “, conclude the authors of the study.
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