Covid-19: compared to the Omicron variant, does the 3rd dose protect more than 90% of severe forms?

Against all expectations, the government gave in to the hypothesis of the vaccination pass, allowing only those vaccinated to access certain establishments and certain activities. While a bill to this effect will be presented on January 5 in the Council of Ministers, the Prime Minister insisted on Friday December 17 on the need to do his booster dose against the Covid.

At a press conference, Jean Castex cited figures to illustrate the vaccine efficacy against the Omicron variant, which seems to be more contagious than all the previous variants: “The first scientific data available to us shows that with two doses, the vaccine appears to be 70% effective against severe forms of Omicron, which is already very important. With three doses or the equivalent, this protective barrier increases sharply. and again exceeds 90%. ”

So what do we know about the resistance of this new variant to current vaccines? And in particular on the risk of developing a severe form of the disease? In reality, there are no exhaustive studies on the characteristics of Omicron (contagiousness and dangerousness) and vaccine efficacy, due to a lack of sufficient cases. But the first results published in South Africa, where the variant was discovered, and in the United Kingdom are encouraging. A study conducted by Discovery Health, a private health insurance company in South Africa, found 211,000 positive PCR tests, of which 78,000 were attributed to Omicron.

On the model of a double vaccination with Pfizer, the study observes an immune escape with Omicron, with only 33% effectiveness against the risk of contamination, but good resistance against severe forms of Covid: the vaccine would protect at 70 % of a risk of hospitalization linked to the disease. Problem, in a country where only 26% of the population is fully vaccinated, according to the Our World in Data platform, and where the booster dose is not even considered for the moment, the only scenario presented here is that of a double vaccination.

To assess the effectiveness of the third dose, we must look at the side of the British who are still ahead of the game. And this for two reasons: Omicron circulates widely in the country (more than 5,000 sequenced tests were already attributed to Omicron the week of December 5, against 30,000 for the Delta, according to the consortium in charge of the sequencing) and 45% of the more than 12 years had already received a booster dose Wednesday, December 15. Enough to give material to researchers wanting to examine the effectiveness of the booster dose against Omicron.

In a study published on December 9 in pre-print (a version not yet validated by its peers), the British Health Security Agency analyzed 581 cases of the Omicron variant and 56,439 cases of the Delta variant. Based on the analysis of these tests, a Pfizer booster dose received two weeks earlier would protect between 71% and 75% of symptomatic forms. But when it comes to the risk of developing a severe form, the number of cases analyzed here is too low to achieve results, say the authors, who are nevertheless optimistic: “It will take a few weeks before we can estimate the effectiveness of Omicron against severe disease, but based on this experience, it is likely to be significantly higher than estimates against symptomatic disease of the variants of concern. SARS-CoV-2 and variants under study in England. “

Another study went further using models based on four different studies, including those from the Pfizer laboratory and the British health security agency. Posted on medRxiv on December 14, a preprints site in medical research, it looks at the effectiveness of the double vaccination and that of the booster dose. In the case of two doses of Pfizer or Moderna vaccine received six months earlier, the risk of symptomatic forms would be reduced to 40% and that of severe forms to 80%. In the case of a booster dose with an mRNA vaccine, the symptomatic forms would be avoided by 86.2% and the severe forms by 98.2%.

“Data on loss of neutralization against the Omicron variant reveal a considerable leakage of neutralizing responses, but indicate that high levels of protection against symptomatic and severe infections are likely to be achieved.” by betting on “existing vaccines that target the protein” Spike, or mRNA vaccines, the researchers conclude. It is these latest results which were cited by Jean Castex during his press briefing on December 17, and which are also found in the opinion of the Scientific Council on the Omicron variant, updated on December 16. However, they deserve to be validated by publication in a scientific journal, but also confirmed by other data from real life.

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