Why are more and more vaccinees infected with the Omicron variant?

In the middle of the fifth wave, more and more people, even vaccinated with their booster dose, are testing positive for Covid-19. Is the contagiousness of the Omicron variant the only reason? Here’s what you need to know.

Since the end of December 2021, daily contaminations have broken records in France with peaks at more than 300,000 cases per day. Unheard of in two years of a global pandemic. At the same time, more than 90% of the population finished his first vaccination schedule by receiving two doses of vaccine and nearly 30 million French people by taking their booster dose. In question, the contagiousness of the Omicron variant, detected in more than three out of four cases.

Omicron’s fault

If studies seem to show that it is less virulent than the Delta variant, Omicron is however more contagious. It would even be the most contagious variant discovered to date. And one of its peculiarities is to weaken the immunity of the human body, acquired by vaccines or a previous infection. Thereby, “The vaccine reduces the risk of contamination, but not absolutely”, explains Philippe Amouyel, professor of public health at the Lille University Hospital, to our colleagues at LCI. In addition, it evokes a protection against infection with Omicron of the order of only “30 to 40%” after the third dose, when it was estimated to be 70% facing the Delta variant and more than 90% against the initial strain of Sars-Cov-2.

For his part, Olivier Véran, Minister of Health, believes that vaccine protection is higher, of the order of “85%”, while admitting that this variant “Is less sensitive to the vaccine than the Delta variant”.

A time-limited “booster”?

If the booster dose is having an effect “booster” on the immunity acquired by the vaccine, this protection seems relatively weak and the duration of this effect is still unknown. Indeed, several researchers have tried to measure the effectiveness of the booster dose against Covid. According to a first study published in December 2021 in Denmark, the triply vaccinated were not 100% protected against infection. But they were however much more than the people who did not realize their recall.

Since then, other studies have come to clarify the previous ones. Thibault Fiolet, epidemiologist at Inserm, explains that “The effectiveness of the third dose against symptomatic infection varies between 54 and 75%”.

This confirms the fears of effective protection, but not complete and therefore protection decreases over time: “According to the latest analysis from Public Health England, the effectiveness of the booster dose decreases a little after ten weeks […] Up to four weeks after the third injection, the protection was 65 to 75%. At ten weeks, that dropped to 40 to 50%. ” This therefore represents a drop in efficiency of more than a third (37%) in two and a half months.

Vaccination as important as ever

Be careful, these studies do not mean that the booster dose is useless, on the contrary. If the vaccines lose their effectiveness against Omicron, they still seem to preserve “More than 95% of severe forms” of the disease, assures Philippe Amouyel. Information repeated by health authorities for several months. “The third dose immediately raises the level of protection well above 90% against the risk of serious forms”, thus assured Olivier Véran, at the end of December.

The question now is whether a fourth dose will soon be necessary to maintain a sufficient protection or if collective immunity provided by the lightning transmission of the Omicron variant will postpone this delay.

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