With the Omicron variant (B.1.1.529), SARS-CoV-2 infections and hospitalizations reached record levels.1 Children younger than 5 years may be especially vulnerable because they are not eligible for COVID-19 vaccination. Wang, et al. (2022) examined incidence rates and clinical outcomes of Omicron infection before and after Omicron became the predominant variant in the U.S.

The study population contained three cohorts of children younger than 5 years with no prior SARS-CoV-2 infection: (1) Omicron cohort, who contracted SARS-CoV-2 infection between December 26, 2021, and January 25, 20224; (2) Delta (B.1.617.2) cohort, who contracted SARS-CoV-2 infection between September 1, 2021, and November 15, 20214; and (3) Delta2 cohort, who contracted SARS-CoV-2 infection between November 16 and November 30, 2021. Delta2 cohort was developed to control for later time periods and shorter infection window.

The researchers examined monthly incidence rates of SARS-CoV-2 infection (new cases per 1000 persons per day) between September 1, 2021, and January 31, 2022, among children without prior infections, stratified by 2 age groups (0-2 and 3-4 years). We tested whether severe clinical outcomes differed between Omicron and Delta cohorts and between Delta2 and Delta cohorts. Cohorts were propensity-score matched for demographics (Table). Risk of death, emergency department visits, hospitalizations, intensive care unit (ICU) admissions, and the need for mechanical ventilation within 14 days after initial SARS-CoV-2 infection were compared between matched cohorts using hazard ratios (HRs) and 95% CIs. Statistical tests were conducted within the TriNetX Analytics Platform with significance set at a 2-sided P value <.05.

This cohort study included a total of 651,640 children younger than 5 years: (1) Omicron cohort, 22,772 children; (2) Delta cohort, 66,692 children; and (3) Delta2 cohort, 10,496 children. The monthly incidence rate of SARS-CoV-2 infections was mostly stable (1.0-1.5 cases per 1000 persons per day) between September and November 2021 (Delta-predominant period) but rapidly increased to 2.4 to 5.6 cases per 1,000 persons per day in December 2021, coincident with the emergence of Omicron variant. Monthly incidence rate of SARS-CoV-2 infections peaked at 8.6 cases per 1000 persons per day in the first half of January 2022 (Omicron-predominant period) and 8.2 in the second half of January 2022. Incidence rate of Omicron infection was higher in children aged 0 to 2 years than in those aged 3 to 4 years. Omicron cohort was younger and with fewer comorbidities than Delta cohort, but differences were eliminated after matching (Table). Risks for severe clinical outcomes in children infected with Omicron variant were significantly lower than those in the matched Delta cohort (Figure, A), whereas the risks for severe clinical outcomes in Delta2 cohort did not differ from those in Delta cohort (Figure, B). There were fewer than 10 deaths in all cohorts.

Reference: Wang L, et al. Incidence Rates and Clinical Outcomes of SARS-CoV-2 Infection With the Omicron and Delta Variants in Children Younger Than 5 Years in the US. JAMA Pediatr. Published online April 1, 2022. doi:10.1001/jamapediatrics.2022.0945