In Children Under 5, Omicron COVID-19 Infection Was More Likely, But Delta Was More Severe

Children under 5 became infected with the Omicron variant 6-8 times more frequently than young children who contracted Delta. However, Delta COVID-19 infections were more severe than Omicron.

When Omicron became responsible for the majority of new COVID-19 infections in December 2021, it was the most infectious variant yet. Children, previously believed to be immune to severe COVID-19, had record levels of infections and hospitalizations.

Children under 5 are not eligible to receive a COVID-19 vaccine, making them particularly vulnerable. A new study, published in JAMA Pediatrics, sought to determine COVID-19 incidence rates and clinical outcomes in children under 5 before and after Omicron became the dominant variant in the US.

The cohort study was conducted from September 1, 2021-January 31, 2022, during which first Delta and then Omicron was the predominant COVID-19 variant. The study population included 3 cohorts of children under 5 years old, none of whom had a prior COVID-19 infection.

The Omicron cohort contracted COVID-19 from December 26, 2021-January 25, 2022, the Delta cohort contracted COVID-19 from September 1, 2021-November 15, 2021, and the Delta2 cohort were infected between November 16-30, 2021.

The study cohorts were further stratified by age group (0-2 and 3-4 years), and were propensity-score matched for demographics, risk of death, emergency department visits, hospitalizations, intensive care unit admissions, and need for mechanical ventilation.

The investigators used the TriNetX Analytics Platform to access the health data of 90 million American COVID-19 patients from 66 healthcare organizations. Patients included in the study represented 28% of the total US population, spanning 50 states and representing diverse age, race, income, and insurance profiles. The investigators examined monthly COVID-19 incidence rates to test how severe clinical outcomes differed among the Omicron, Delta, and Delta2 cohorts.

The final study population was comprised of 651640 children under 5 years of age, 22772 in the Omicron cohort, 66692 in the Delta cohort, and 10496 in the Delta2 cohort. During the Delta period, the COVID-19 incidence rate remained relatively stable. However, as Omicron emerged in December 2021, cases rapidly increased from 1.0-1.5 to 2.4-5.6 cases per 1000 persons per day. At the height of the Omicron wave in the first half of January 2022, monthly COVID-19 incidence rate peaked at 8.6 cases per 1000 persons per day.

The cohort of children who contracted Omicron was younger and had fewer comorbidities than either of the Delta cohorts; the incidence rate of Omicron infection was higher in children 0-2 years than children 3-4 years of age. However, these differences were null after matching the cohorts. The risk of severe clinical outcomes in the Omicron cohort were significantly lower than the matched Delta cohorts. In all 3 cohorts of infected children under 5, there were fewer than 10 deaths.

The investigators concluded that children under 5 years old contracted Omicron 6-8 times more frequently than they contracted Delta. Notably, however, severe clinical outcomes were less common in the Delta cohorts.