At a time when the epidemic vice of Covid-19 seems to be loosening after a month of constant reflux of the Omicron wave carried by the BA.1 sub-variant, should we fear a rebound caused by its “little brother” BA.2 , whose progression is confirmed from week to week in France? The models published on Wednesday February 23 by the team of Simon Cauchemez, at the Institut Pasteur, show the horizon for the coming weeks: “The BA.2 epidemic is slowing down the decrease in cases without however generating a significant epidemic rebound”, write the modellers.
A reassuring result which thwarts the catastrophic scenarios that could have been feared by the greater transmissibility of this sub-lineage of Omicron endowed with a dozen additional mutations and which caused an epidemic rebound in Denmark at the very time when the country was abandoning its sanitary measures. According to Pasteur’s team, BA.2 is indeed 70% more transmissible than the other sub-variants of Omicron, whose original strain, BA.1, was already estimated to be much more transmissible than the majority Delta variant. in France between July and December 2021.
“We are not waiting for the epidemic rebound that we have had in certain countries, such as Denmark, because this increase in BA.2 occurs quite late compared to the Omicron wave”, explains Simon Cauchemez, head of the mathematical modeling of infectious diseases unit at the Institut Pasteur. Based on flash sequencing surveys conducted weekly by Public Health France and the continued increase in BA.2 cases, researchers expect half of Omicron cases to be BA.2 infections around of February 24. Within a month, on 1er April, about 35% of the French population could have been infected with an Omicron virus, whether BA.1, BA.2 or BA.3, the three sub-lineages identified so far.
An incidence rate at unprecedented levels
The key mechanism at work here is the very strong immunity acquired by the French population during the fifth wave thanks to infections and vaccination. “For the first time since the start of the pandemic, the very rapid decrease in infections is not due to the strict control measures but to the level of immunity”, the researchers point out. A first allowed in particular by the lesser severity of this variant compared to the previous ones.
“The hypotheses that we make are quite solid in the short term: when we have been infected with Omicron, we are a priori well protected against reinfection with BA.2”, advances Simon Cauchemez. However, the wave of infections was so important, bringing the incidence rate to levels never reached before – more than seven times higher than the peak of the second wave – that the French are well protected against BA.2 .
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