First and foremost, it is important to clarify the context at the time of the survey. The Delta variant was still the dominant variant in the country. We could see that Omicron cases were rising in other countries, and had begun to detect the first cases in France.

While the data available in the testing (SI-DEP), hospitalization (SI-VIC), and vaccination (VAC-SI) databases provide a wealth of information for monitoring the epidemic, they remain fairly limited regarding case characteristics, largely due to difficulties in matching them in real time and the absence of clinical details. In collaboration with the regional units (CR) of Santé publique France, we quickly decided to collect detailed information—not available in these databases—on the first cases. Our goal was to characterize the Omicron variant in order to anticipate its potential impact on the population and the healthcare system.

The challenge was to gather all this information in a very short time within the difficult context of the pandemic and the rapid increase in the number of cases. Knowledge of the variant was made available through the sequencing of confirmed cases by the EMERGEN consortium (see Box). Each person identified as a confirmed Omicron case was contacted by epidemiologists from SpFrance’s regional units to complete a standardized questionnaire. The information collected included travel history, clinical symptoms, comorbidities, prior SARS-CoV-2 infections, and vaccination status. During the study period (November 23, 2021, to January 11, 2022), 468 Omicron cases spread across nearly the entire country (17 of the 18 regions) were investigated.

Our investigation of these first 468 Omicron cases in France showed that, while the majority (89%) of cases were symptomatic, the symptoms were mild in the vast majority of cases. Loss of smell and taste—a very common symptom in infections caused by other variants—was reported only rarely (8.3% and 9%, respectively). This is an important finding because, until now, it has been one of the main symptoms specifically suggesting a SARS-CoV-2 infection. Among the cases investigated, only seven (2%) were hospitalized; none were admitted to intensive care or died. However, we must remain cautious in interpreting the results, as our study focused on a population of relatively young individuals (median age: 35 years) and a majority (64% of cases) were fully vaccinated, 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. For instance, a study recently published by Santé publique France comparing the respective severity of these two variants showed 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 the variants diminished with age. Specifically, this risk was reduced by a factor of 9.1 among people aged 40 to 64 infected with Omicron, by a factor of 5.3 among those aged 65 to 80, and by only a factor of 2.0 among those aged 80 and older (3).

When a new variant such as Omicron emerges, rapid detection and characterization of cases are essential.

This helps determine whether the new variant causes more severe or different symptoms than previous ones, or leads to more hospitalizations or deaths. The question of the effectiveness of current vaccines against emerging strains is also crucial. All this information is essential for decision-making regarding healthcare capacity and organization, testing strategies, and control measures.
The French public health system, including the screening and genomic surveillance laboratories united within the EMERGEN consortium (see Box), and local, regional, and national authorities, have demonstrated their ability to respond and adapt rapidly to the emergence of a new variant. Since February 18, 2022, a Delta/Omicron recombinant (to which no lineage name has yet been assigned) has been subject to enhanced monitoring by the consortium’s laboratories. The majority of its genome corresponds to the Delta variant (sublineage AY.4), but a large portion of the S gene (encoding the spike protein) corresponds to the Omicron variant (sublineage BA.1). As of March 8, 27 sequences of this recombinant, which has been circulating at low levels for several weeks, had been detected in France. To date, very little data is available on its characteristics, and investigations are ongoing.