Omicron Variant in the Critical Care Units of the Paris Metropolitan Area: The Reality Research Group

A new severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variant, named Omicron, has recently been reported (1, 2). A reduction in the relative risk of hospitalization for subjects with Omicron infection was shown (3, 4), and subjects with Omicron infection were also less likely to be admitted to ICUs and to receive mechanical ventilation (5). Most of these results were based on administrative databases. The aims of our study were to compare the risk of ICU admission among Deltaand Omicron-infected patients who were admitted to Assistance Publique des Hôpitaux de Paris (APHP) between December 1, 2021, and January 18, 2022, and to compare the characteristics of those patients admitted to its ICUs during the same period. This study was approved by the ethics and scientific committees of APHP (CSE-21-32). The number of patients admitted to emergency departments, medical wards, and ICUs was collected on a daily basis from the Syst eme d’Information pour le Suivi des Victimes (SI-VIC) database, which provides real-time data on patients with coronavirus disease (COVID-19) hospitalized in France. All consecutive patients with COVID-19 admitted to ICUs of the APHP group have been registered in the Reality clinical database since July 2020. Some of the results of these studies have been previously reported in preprint form (https://doi.org/10.1101/2022.01.25. 22269839). RT-PCR positivity for SARS-CoV-2 was collected from the APHP laboratory information system. The screening test analyzed the following mutations in the spike protein: L452R (mutation C) and D69-70 or N501Y, or K417N (mutation D). In the study period, no a, b, or g variant was detected by sequencing. Therefore, C1D0 corresponds to a Delta variant, and C0D1 corresponds to an Omicron variant (6, 7). Only patients who received no doses of vaccine were considered unvaccinated. Causes of immunosuppression were reported as published (8, 9). Patients were treated according to the APHP guidelines. Only patients hospitalized for COVID-19 were included, as patients with incidental COVID-19 were individualized in the SI-VIC database and not studied and were not included in Reality. The risk of being admitted to the ICU for Omicron and Delta infection was calculated from the SI-VIC administrative database. Characteristics and outcomes of critically ill patients with Omicron and Delta infection were determined from the Reality clinical database. Comparisons were performed using the Wilcoxon test for continuous variables and the chi-square test for percentages. Adjusted analyses were performed using survival analysis in the subgroup of patients hospitalized in ICUs for pneumonia, with in-ICU mortality as the event and ICU discharge alive as the censoring variable. Hazard ratios (HRs) for variables were then reported. Statistical analysis was conducted in R (R Core Team). We identified 5,140 patients in the hospitals of the APHP group from SI-VIC, of whom 1,379 (26.8%) lacked variant information. Among the 3,761 patients positive for either the Delta (n=1,376) or Omicron (n=2,385) variant, 579 (15.4%) were hospitalized in ICUs, 287 (7.6%) initially and 292 after transfer among the 1,628 patients (17.9%) hospitalized in the ward. The absolute risk of ICU admission with Omicron was 16% less (95% confidence interval [CI], 14.1–19.3%) than with Delta (9.3% vs. 25.8%; P, 0.001), with a 64% reduction. Eight hundred eighty-eight patients were included in Reality (Table 1), 517 of them (58.2%) unvaccinated and 302 (34%) with two or three doses of vaccine. Among the 629 patients with variant information, 400 (63.6%) had Delta variants and 229 (36.4%) had Omicron variants. Compared with Delta-infected patients, those with Omicron infection were more frequently immunocompromised (P, 0.001) and more frequently vaccinated (P, 0.001) with at least one dose (57.7% vs. 30.1%), two doses (25.8% vs. 14.5%), and even three doses (26.2% vs. 10.8%). BNT162b2 or mRNA-1273 was used in 83.5% of cases, with no difference between the variants. Pneumonia as a cause of hospitalization was less frequent for Omicron than for Delta (67.2% vs. 94.8%; P, 0.001). For the Omicron variant, pneumonia was less frequent among vaccinated (62.1%) than unvaccinated patients (80.7%; P, 0.001), whereas among the 60 Omicron-infected patients who received three doses of vaccine, 43 (71.7%) were admitted for pneumonia. Subjects with Omicron infection were less frequently invasively ventilated compared with those with Delta infection (41.0% vs. 51%, respectively; P = 0.02). Table 2 reports characteristics of both variants in the subgroup of patients admitted for pneumonia. Similar results were observed, even though patients with Omicron were older and more frequently admitted to the ICU in the 5 days after symptom onset (P = 0.004). They were still less likely to be invasively ventilated (39% vs. 50.4%; P = 0.021). Among patients discharged from the ICU on January 18, 2022 (80% of Delta cases and 63.6% of Omicron cases), unadjusted in-ICU mortality between variants did not differ in the overall population (20.0% and 27.9%, respectively; P = 0.08; Table 1) and in the subgroup of pneumonia (31.6% vs. 29.7%, respectively; P = 0.78; Table 2). After adjustment, variant (Omicron vs. Delta) was not associated with mortality in patients with pneumonia (HR, 1.34; 95% CI, 0.82–2.17; P = 0.24). Other variables included in the model were age above the median (HR, 2.31; 95% CI, 1.52–3.5), time from symptom onset to ICU admission (6–8, 9–11, 12–14, and >15 d vs. <5 d; HRs, 0.44 [95% CI, 0.27–0.71], 0.39 [95% CI, 0.23–0.67], 0.34 [95% CI, 0.16–0.73], and 0.34 [95% CI, This article is open access and distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives License 4.0. For commercial usage and reprints please contact Diane Gern (dgern@thoracic.org).