Covid-19 vaccine: from Pfizer to Moderna – Sciences et Avenir

Are you taking a risk by receiving Moderna for the third dose after being vaccinated with Pfizer? With the urgency of stopping the fifth wave, the booster dose has become a national priority. But due to the lack of doses of Pfizer for all French eligible for this third dose, many were offered a dose of Moderna instead. Given the increased risk of myocarditis in people younger than 30 years after injecting this vaccine (cases that, however, remain rare and have a favorable course), it is legitimate to ask the question: is it a good idea to switch to Moderna ? Sciences et Avenir takes stock.

A matter of dosage

The RNA vaccines from Pfizer and Moderna use the same technology, that is, fragments of RNA that will then generate pieces of the spike protein of the coronavirus. These bits of protein will teach the immune system to recognize the coronavirus, so it can neutralize it during an infection. Because of these similarities, the two vaccines can cause similar side effects, such as this rare risk of myocarditis in young people. However, it is a bit higher for Moderna, but this is not due to a difference in the design of the vaccine, but rather a question of dosage.

In fact, one dose of Moderna’s vaccine contains 100 micrograms of RNA compared to Pfizer’s 30. Three times the RNA that generates a greater immune response with Moderna, which can cause a little more adverse reactions. To avoid this increased risk, Moderna’s dose was lowered to the booster dose, at just 50 micrograms, closer to Pfizer’s 30.

Milder effects after a Moderna boost, but no serious effects.

However, no increased risk is seen after Moderna, even when the full dose (100 micrograms) is administered. This is the conclusion of a study published in English on December 1 in The Lancet, which analyzed the effect of the booster dose in almost 3,000 people older than 30 years. About 200 of those who were vaccinated with Pfizer for the first two doses received a full dose of Moderna or a half dose of Pfizer.

About half of the people in these groups reported side effects (42/110 for Pfizer vs 50/110 for Moderna), the severity of which was mild in about 75% in both groups (mainly headaches and fatigue). None of these subjects developed heart problems after the booster dose, and a minority reported vascular problems (2.4% in Pfizer, 6% in Moderna). In Pfizer’s group, one person was hospitalized for a liver problem and another for nervous system problems (the researchers conclude that the link to vaccination is unlikely in both cases). In Moderna’s group, one person was hospitalized due to myocardial ischemia, but according to the authors there was no relationship to the booster dose (the cardiac complication occurred on the same day as this dose, making it highly unlikely that there is a booster dose). causal relationship).

The authors conclude that although the Moderna booster dose may cause further discomfort (arm pain, headache, and fatigue), mainly in people aged 30 to 69, there is no difference in serious side effects between the two vaccines. RNA.

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