COVID infection severity in children under 5 years old before and after Omicron emergence in the US (preprint)

ABSTRACT

Abstract Importance Pediatric SARS-CoV-2 infections and hospitalizations are rising in the US and other countries after the emergence of Omicron variant. However data on disease severity from Omicron compared with Delta in children under 5 in the US is lacking. Objectives To compare severity of clinic outcomes in children under 5 who contracted COVID infection for the first time before and after the emergence of Omicron in the US. Design, Setting, and Participants This is a retrospective cohort study of electronic health record (EHR) data of 79,592 children under 5 who contracted SARS-CoV-2 infection for the first time, including 7,201 infected when the Omicron predominated (Omicron cohort), 63,203 infected when the Omicron predominated (Delta cohort), and another 9,188 infected when the Omicron predominated but immediately before the Omicron variant was detected in the US (Delta-2 cohort). Exposures First time infection of SARS-CoV-2. Main Outcomes and Measures After propensity-score matching, severity of COVID infections including emergency department (ED) visits, hospitalizations, intensive care unit (ICU) admissions, and mechanical ventilation use in the 3-day time-window following SARS-CoV-2 infection were compared between Omicron and Delta cohorts, and between Delta-2 and Delta cohorts. Risk ratios, and 95% confidence intervals (CI) were calculated. Results Among 7,201 infected children in the Omicron cohort (average age of 1.49 years), 47.4% were female, 2.4% Asian, 26.1% Black, 13.7% Hispanic, and 44.0% White. Before propensity score matching, the Omicron cohort were younger than the Delta cohort (average age 1.49 vs 1.73 years), comprised of more Black children, and had fewer comorbidities. After propensity-score matching for demographics, socio-economic determinants of health, comorbidities and medications, risks for severe clinical outcomes in the Omicron cohort were significantly lower than those in the Delta cohort ED visits 18.83% vs. 26.67% (risk ratio or RR 0.71 [0.66-0.75]); hospitalizations 1.04% vs. 3.14% (RR 0.33 [0.26-0.43]); ICU admissions 0.14% vs. 0.43% (RR 0.32 [0.16-0.66]); mechanical ventilation 0.33% vs. 1.15% (RR 0.29 [0.18-0.46]). Control studies comparing Delta-2 to Delta cohorts show no difference. Conclusions and Relevance For children under age 5, first time SARS-CoV-2 infections occurring when the Omicron predominated (prevalence >92%) was associated with significantly less severe outcomes than first-time infections in similar children when the Delta variant predominated.