Fatigue, headaches, chills, fever… These side effects of anti-covid vaccines are common. And with the second injection, they occur even more often than with the first.

This is what theNational Medicines Safety Agency (ANSM) on its website, about Pfizer and Moderna messenger RNA vaccines: “the frequency of systemic reactions, especially fever, fatigue and headache, is greater during the administration of the 2nd dose compared to the 1st dose“.

“More intense” effects

More frequent, but also more intense, underline for their part the Centers for the control and the prevention of the diseases (CDC) American: “The side effects after your second injection may be more severe than those you experienced after your first injection.. “

Read also: Infection or vaccination, the same immunity?

Immune reaction

How to explain it? First of all, you should know that these effects are normal, and indicate that your body is responding to the vaccine and building immunity. They usually go away within a few days.

Their occurrence is explained by the very principle of the vaccine, which mimics the entry of a virus into our body. Symptoms similar to those of the viral disease then appear, but are of course weaker, benign and shorter than the real symptoms.

“Boost” of the second dose

With the first dose, it takes a while for the body to identify the “enemy” of the vaccine. But at the second dose, he already knows it and therefore recognizes it quickly.

The immune reaction is then faster and stronger, as are the possible side effects. This is what scientists call the immune “boost”: the second dose amplifies and strengthens the immunity built in the first dose.

Every immune system is different

In the case of people who have already had covid and who only need a single dose of vaccine, it is this first and only dose that acts as an immune “boost”.

A “first” dose may therefore cause more side effects in people who are already immune than in those who have never been in contact with the disease.

Last point to know: the absence of side effects does not mean that the vaccine did not work. Every immune system responds differently, and immunity can be built without your body showing noticeable symptoms.