Is the Omicron variant already in the majority in Ile-de-France?

As we learn more about him, we fear his arrival. Since Sunday, December 19, Internet users have been worried about the meteoric advance of the Omicron variant, and in particular in Île-de-France. Supporting figures, some even assure that this variant initially spotted in South Africa has become the majority in the Paris region. What is it really?

As Public Health France reminds us in a document on the risks associated with emerging variants, a “increased surveillance has been set up in France in order to monitor in a more sensitive and reactive manner” the spread of the Omicron variant in the territory. To do this, we do not target it strictly speaking, Omicron does not wear “none of the mutations included in the screening strategy currently deployed in France”. On the contrary, it is the absence of one of these three mutations sought by screening – the L452R – that we arrive at. “suspect this variant”.

It is by following this method that the national public health agency can find out and put online the number of tests for which the presence of this variant is suspected. However, the latest figures do show a worrying explosion of this variant, and in particular in Île-de-France. From December 10 to December 16, the L452R mutation was absent from 19.3% of the tests. That is to say a sevenfold increase in the number of cases compared to the previous week (the Omicron variant then represented 2.9% of the tests).

An even more striking phenomenon in the capital. Over the same period, this mutation was absent from more than a quarter of the tests (27.2%). A week before, this concerned only 3.7% of screenings.

Undeniably, the Omicron variant is experiencing very rapid growth in the Paris region … However, these official figures do not show that it is already in the majority in the population. Except that these data have several biases. Starting with the fact that they only represent a sample of all the tests carried out in the capital. Today, health authorities analyze less than a fifth (16%) of PCR tests, and do not sequence antigenic tests at all, as we have already explained to you. here. While researchers and health authorities agree that this sample is sufficient to reflect the progress of a variant, it may still have certain margins of error.

Moreover, another major problem with these figures is their delay. Public health France only offers data “smoothed” over a week, that is to say by adding up the last seven rolling days. However, this process causes a delay of three days on feedback from the laboratories. The last updated data goes back to last Thursday.

On the other hand, this technique causes an underestimation of the most recent figures, impacted by those at the start of the week, as Florence Débarre explains to us. A researcher at the CNRS, she pleads for daily updating of data so as not to “miss take off” of this variant. Especially since if having daily data makes it possible to smooth the figures, the reverse is impossible. It is therefore a “loss of information”, she regrets.

An engineer therefore tried to find a calculation to overcome this lack of information. To do this, as he explains here, he made a point of evaluating the values ​​for each date based on public data from Public Health France. What to give an approximate idea of ​​the daily results. This method thus made it possible to estimate this rate at 12.5% ​​on Monday the 13th in Île-de-France, when the smoothed data said 8.7%.

A figure which is only an estimate, but which corresponds to what we know about the Omicron variant. The health authorities in the United Kingdom have thus alerted to this variant whose cases “double every two or three days”. With a variant already present on a quarter of the tests last week, it would not be surprising that Omicron took advantage of the weekend to become the majority. However, we will have to wait until the middle of the week to be sure.

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