Rarely has the Scientific Council expressed itself with… an exclamation point. In a “warning note”, made public on Tuesday, the body responsible for guiding the government on the decisions to be taken to counter the Covid-19 epidemic, immediately warns: “The epidemic is not over!”. For the past few weeks, France has observed a sharp rise in daily cases of coronavirus, and the Scientific Council evokes several explanations for this. On the one hand, the anticipation of the relaxation of protective measures plays a role, such as protective immunity against infection which is falling, or even the successive back to school seasons, but this increase is also driven by a sub-variant of Omicron, called BA.2, he notes.
This sub-lineage of the Omicron variant now represents the majority of new infections in France. According to figures given by Public Health France, its share has continuously increased to reach more than 50% of new cases from March 1. Above all, it would be around one and a half times more contagious than its “cousin”, the BA.1, which was at the origin of the explosion of new cases observed from January.
- Higher contagiousness compared to BA.1
In Denmark, one of the first countries to see a sharp rise in cases caused by BA.2, the Statens Serum Institute in Copenhagen calculated that this variant is 30% more transmissible than the previous lineage. Greater contagiousness is now observed wherever BA.2 has taken hold. Thus, the WHO, which was interested in its growth in countries that actively participate in the publication of sequencing data of the virus, showed that in 43 countries observed, and when the two sub-lineages were circulating, the BA. 2 always had the advantage in terms of growth over the BA.1. “In Denmark, the BA.2 actually made it possible to play the extensions”, explains the epidemiologist at the Institute of Global Health in Geneva, Antoine Flahault. “It will find casualties in the population, but this subvariant didn’t really spawn a ‘new wave’, it just extended the wave beyond what was expected.”
In a document published mid-February the UK Health Security Agency (UKHSA) also notes that the time between two infections in a chain of infection, known as the “serial interval”, “is about half a day less for BA .2 than BA.1”. The mean serial interval is 3.27 days for BA.2 compared to 3.72 days for BA.1. Clearly, infections spread more quickly during a wave of contamination, and this could contribute to “the increase in the growth rate of BA.2”, notes the UKHSA.
- Low risk of reinfections
One of the concerns with this sub-variant was also the possibility of re-infections. The first data are rather reassuring: several studies have shown that cases of reinfections were marginal in new contaminations with BA.2. In its document, the British health security agency recorded only 274 cases of reinfections out of the 3,581 cases of people carrying BA.2, or just over 7% of cases. What is the risk of people who have been infected with BA.1 becoming infected with BA.2 again? “It remains very rare,” tempers Olivier Schwartz, who heads the Virus and Immunity Unit at the Institut Pasteur. The British experts thus showed “that there was no case of reinfection with BA.2 documented by sequencing after an infection with BA.1”, among the cases studied. “It is estimated that about 40% of the population has been contaminated by Omicron in the last few weeks, but this was due to the BA.1 strain. Now that BA.2 has become the majority in new contaminations, it manages to affect people who had not been infected by Omicron at the time”, explains Olivier Schwartz.
This situation is not, however, immutable, warn certain epidemiologists. Due in particular to the decline over time of the protection conferred by an infection, or the vaccine. “Just after the BA.1 peak, reinfections are very rare, but over time immunity to infection is less and less important, and therefore the possibility of reinfection greater. This is true in particular for the most benign infections, which induce weaker immunities”, anticipates the epidemiologist and specialist in the modeling of infectious diseases at the University of Montpellier Mircéa Sofonéa.
In its latest note, the Scientific Council makes the same observation, commenting on a model establishing a scenario where the number of cases would reach 100,000 daily cases by March, it writes: “This model makes the strong assumption that people infected with Omicron are protected against reinfections in the short term (i.e. until the end of the simulations on April 1. There is probably good protection in the short term, but it is likely that this protection fades fairly quickly. This assumption could lead the model to be overly optimistic, especially for those infected/vaccinated at the end of 2021.”
- No more serious forms with BA.2
Good news about this sub-line of Omicron is that it would not cause more severe form than its “cousin” BA.1. Early preliminary data from the UK and Denmark show that the risk of hospitalization is the same with BA.2 as with BA.1. A finding shared by the WHO: “We do not see a difference in terms of severity between BA.1 compared to BA.2 and therefore it is a similar level of severity in terms of the risk of hospitalization”, had thus declared at the end of February Maria Van Kerkhove, who oversees the fight against Covid-19 at the WHO. Moreover, even if the vaccines are affected by the Omicron variant, they do not now seem to demonstrate a loss of efficacy between the two sub-variants.
- A variant that has less of an impact on certain monoclonal antibodies
In the same way that the BA.1 variant of Omicron had caused a loss of efficacy on certain treatments, several monoclonal antibodies are directly affected by this new variant. These synthetic antibodies, created in the laboratory from the natural response of a person infected with the coronavirus, can be injected into people in whom vaccination does not generate a sufficient immune response. A recent study by the Institut Pasteur, available in preprint, thus shows that several treatments would have lost effectiveness. “If we look at the overall picture, many neutralizing antibodies no longer work in the face of Omicron mutations. But it is interesting to see that the two sub-variants BA.1 and BA.2 do not escape the same neutralizing antibodies” , explains Timothée Bruel, researcher at the Institut Pasteur within the Virus and Immunity team, and who participated in this study.
These results, which are still preliminary and which are based on in vitro data, thus show that if certain monoclonal antibodies, such as Ronapreve, which were already known to be ineffective against BA.1, remain very affected by BA.2, d Others, like the Evusheld, seem to be less affected by this sub-lineage. “With Evusheld, the neutralization of BA.1 was detected in 19 out of 29 patients, whereas this treatment allowed the neutralization of BA.2 in all of the patients”, details Thimothé Bruel. “Both BA.1 and BA.2 have very large mutations in the same region of the spikes protein, but these mutations are unique between the two subvariants and lead to different antibody responses,” explains the researcher.
Faced with this new form of the virus, and the rise in cases, the Scientific Council insists on the importance of the conservation of protective measures in the oldest, fragile, and immunocompromised and notes that “we do not yet know the frequency and severity of long COVIDs that could be associated with it”.