First of all, it is important to describe the context at the time of the survey. The Delta variant was still the dominant strain in the country. We could see that Omicron cases were increasing in other countries, and the first cases had begun to be detected in France.

Although the data available from testing (SI-DEP), hospitalization (SI-VIC), and vaccination (VAC-SI) databases provide a wealth of information for monitoring the epidemic, they remain rather limited regarding the characteristics of the cases due to the difficulty of performing real-time matching and the absence of clinical details. In collaboration with the regional units of Santé publique France, we quickly decided to collect further details about the first cases that were not available in these databases. Our objective was to characterize the Omicron variant in order to anticipate its potential impact on the population and the healthcare system.

The challenge was to collect all this information in a very short time, amid the difficult context of the pandemic and the rapid rise in case numbers. Knowledge about the variant was made available through the sequencing of confirmed cases by the EMERGEN consortium (see box). Each person confirmed to have the Omicron variant was contacted by epidemiologists from regional units of Santé publique France to complete a standardized survey. The collected information included travel history, clinical symptoms, comorbidities, previous SARS-CoV-2 infections, and vaccination status. In this way, 468 Omicron cases were investigated during the study period (from November 23, 2021, to January 11, 2022), covering nearly the entire country (17 out of 18 regions).

Our investigation of these first 468 cases of Omicron in France showed that, although most cases (89%) were symptomatic, symptoms were mild in the vast majority of cases. Loss of smell and taste, which are very common with other variants, were reported only rarely (8.3% and 9%, respectively). This was a significant finding, as these symptoms had previously been among the main indicators of SARS-CoV-2 infection. Among the cases investigated, only 7 (2%) were hospitalized, and none were admitted to the intensive care unit or had died. However, we must remain cautious when interpreting the results because our study was based on a population composed of relatively young individuals (median age: 35 years) and a majority (64% of cases) had received two doses of the vaccine, which also provides some protection against severe forms of the disease.

Since our study, several publications have shown that Omicron appears to have a reduced impact on the healthcare system compared to Delta. In this regard, a study recently published by Santé publique France comparing the respective severity of these two variants concluded that the risk of experiencing a serious hospital event was lower for people infected with Omicron than for those of the same age infected with Delta. However, this difference between variants diminished with age: this risk was reduced by a factor of 9.1 for people aged 40–64 years infected with Omicron, by a factor of 5.3 for those aged 65–80 years, but only by a factor of 2.0 for those aged 80 years and older (3).

When a new variant such as Omicron emerges, rapid detection and characterization of cases is essential. This helps determine whether the new variant causes more severe or different symptoms than previous variants, as well as whether it leads to more hospitalizations or deaths. The effectiveness of current vaccines against emerging strains is also a crucial question. All this information is important for making decisions regarding healthcare capacity and organization, screening strategies, and control measures.

The French public health system, including the screening and genomic surveillance laboratories brought together by the EMERGEN consortium (see box), alongside local, regional, and national authorities, have collectively demonstrated their ability to respond and adapt rapidly when a new variant emerges. Since February 18, 2022, a Delta/Omicron recombinant now known as XD has been under close observation by the consortium laboratories. Most of its genome corresponds to the Delta variant (sublineage AY.4), but a large portion of the S gene (coding for the Spike protein) corresponds to the Omicron variant (sublineage BA.1). On March 8, 2022, 27 sequences of this recombinant, which has been spreading at low levels for several weeks, were detected in France. To date, very little data is available on its characteristics. Investigations are ongoing.