Higher incidence of SARS-CoV-2 infection in under 5s during omicron

For children aged under 5, the incidence rate of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection was higher with the omicron variant versus the delta variant, according to a research letter published online April 1 in JAMA Pediatrics.

Lindsey Wang, from the Case Western Reserve University School of Medicine in Cleveland, and colleagues examined incidence rates of SARS-CoV-2 infection in three cohorts of children aged younger than 5 years with no prior SARS-CoV-2 infection: the omicron cohort (22,772 children), who contracted SARS-CoV-2 between Dec. 26, 2021, and Jan. 25, 2022; the delta cohort (66,692 children), who contracted SARS-CoV-2 between Sept. 1 and Nov. 15, 2021; and the delta2 cohort (10,496 children), who contracted SARS-CoV-2 between Nov. 16 and Nov. 30, 2021.

The researchers found that the monthly incidence rate of SARS-CoV-2 infections was mostly stable between September and November 2021 (1.0 to 1.5 cases per 1,000 persons/day), but increased to 2.4 to 5.6 cases per 1,000 persons/day in December 2021, coincident with omicron emergence. The monthly incidence rate of SARS-CoV-2 peaked at 8.6 and 8.2 cases per 1,000 persons/day in the first and second half of January 2022, respectively. Risks for severe clinical outcomes were significantly lower for children infected with omicron variant versus the matched delta cohort; the risks for severe clinical outcomes did not differ for the delta and delta2 cohorts.

"Study findings may inform risk-benefit considerations about in-person school attendance, mask use, and vaccination implementation for young children," the authors write.