Until now, there have been no studies on the subject.
By publishing the results of an experiment conducted in February and March by English researchers on Wednesday, the British scientific journal The Lancet has partially filled the void around this question: what can be the consequences of a mixture of vaccines against Covid-19 between the 1st and the 2nd injections?
According to this study, the results of which were based on three blood tests and participants' self-reported symptoms, those who received two different vaccines experienced more intense short-term side effects than groups who received the same serum in both vaccines. injections. Indeed, 34% of the 110 recipients of AstraZeneca in the first dose and Pfizer as a booster say they have a fever, against only 10% of the 112 recipients of AstraZeneca alone. This symptom is even more present for groups who received Pfizer for the first injection.
"Similar increases were seen for chills, fatigue, headaches, discomfort, and joint or muscle pain," observe the study authors.
However, the consequences of the mixture of vaccines seem to end there: "There has been no hospitalization due to solicited symptoms, and most of this increase in reactogenicity
(excessive immunological response to a vaccine, Editor's note)
has observed within 48 hours of immunization ”, not beyond.
Scientists feared unforeseen reactions
With the messenger RNA vaccine being used for the first time in humans, scientists feared unforeseen reactions.
“As the immune stimulation is not the same, with each injection the immune system is surprised, so it reacts violently twice, explains Stéphane Gayet, infectious disease specialist at Strasbourg University Hospital.
In a way, it's reassuring: it shows us that the immune system is reactive ”.
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In younger people, these side effects could be even more intense, warn the British researchers, as these data were obtained in participants aged 50 and over.
However, mixed vaccination concerns young people in France, where vaccination with AstraZeneca has been reserved for over 55s since March 19.
This is also the case in Germany and Sweden (reserved for over 65s).
As for Norway and Denmark, they have simply given up on the use of the Swedish-British vaccine: in these countries, mixed vaccination therefore affects all those who have received a first dose of AstraZeneca.
Short-lived "reassuring" reactions
"Regardless, it is reassuring that all symptoms were short-lived and that the limited hematology and biochemistry data available were not of concern," conclude the study authors.
Other similar experiments including Moderna and Novavax vaccines are underway.
Furthermore, it is too early to compare the effectiveness of a mixed vaccination and a homogeneous vaccination against Covid-19.
“The researchers carried out similar experiments in mice, and by injecting different vaccines, we even found that the immunity was of better quality, more effective, notes Stéphane Gayet.
But in humans, we do not yet know.
"