Clinical outcomes of the severe acute respiratory syndrome coronavirus 2 Omicron and Delta variant: systematic review and meta-analysis of 33 studies covering 6 037 144 coronavirus disease 2019–positive patients

BACKGROUND: Although the SARS-CoV-2 Omicron variant is considered to induce less severe disease, there have been no consistent results on the extent of the decrease in severity. OBJECTIVES: To compare the clinical outcomes of COVID-19–positive patients with Omicron and Delta variant infection. DATA SOURCES: Searches were implemented up to 8 November 2022 in PubMed, Web of Science, BioRvix, and MedRvix. STUDY ELIGIBILITY CRITERIA: Eligible studies were cohort studies reporting the clinical outcomes of COVID-19–positive patients with Omicron and Delta variant infection, including hospitalization, intensive care unit (ICU) admission, receiving invasive mechanical ventilation (IMV), and death. PARTICIPANTS: COVID-19–positive patients with Omicron and Delta variant infection. ASSESSMENT OF RISK OF BIAS: Risk of bias was assessed employing the Newcastle-Ottawa Scale. METHODS OF DATA SYNTHESIS: Random-effect models were employed to pool the ORs and 95% CIs to compare the risk of clinical outcome. I(2) was employed to evaluate the heterogeneity between studies. RESULTS: A total of 33 studies with 6 037 144 COVID-19–positive patients were included in this meta-analysis. In the general population of COVID-19–positive patients, compared with Delta, Omicron variant infection resulted in a decreased risk of hospitalization (10.24% vs. 4.14%, OR = 2.91, 95% CI = 2.35–3.60), ICU admission (3.67% vs. 0.48%, OR = 3.64, 95% CI = 2.63–5.04), receiving IMV (3.93% vs. 0.34%, OR = 3.11, 95% CI = 1.76–5.50), and death (2.40% vs. 0.46%, OR = 2.97, 95% CI = 2.17–4.08). In the hospitalized patients with COVID-19, compared with Delta, Omicron variant infection resulted in a decreased risk of ICU admission (20.70% vs. 12.90%, OR = 1.63, 95% CI = 1.32–2.02), receiving IMV (10.90% vs. 5.80%, OR = 1.65, 95% CI = 1.28–2.14), and death (10.72% vs. 7.10%, OR = 1.44, 95% CI = 1.22–1.71). CONCLUSIONS: Compared with Delta, the severity of Omicron variant infection decreased.