Omicron Variant Was More Infectious But Less Severe for Young Children

The bad news is that young children were much more likely to become infected with the Omicron variant of SARS-Cov-2 than the previous Delta variant. The good news, however, is that Omicron infection appeared to be considerably less severe in that age group, according to a large study. Now, the BA.2 sublineage of the Omicron variant is becoming dominant in the United States and is considered even more infectious, although not more serious. Here are more details.

CLEVELAND — Children younger than 5-year-olds were 6 to 8 times more likely to become infected with the Omicron variant of COVID-19 than the previous Delta variant but were less likely to have severe clinical outcomes, according to a new study.

Research from Case Western Reserve University (CWRU) School of Medicine is touted as the first large-scale effort to compare the health outcomes in children too young to be vaccinated. Results were published in JAMA Pediatrics.

Despite Omicron’s much higher infection rate, researchers report that severe clinical outcomes ranged from a 16% lower risk for emergency room visits to an 85% less risk for mechanical ventilation. About 1.8% of children infected with Omicron were hospitalized, compared to 3.3% with Delta, they add.

In terms of infection rate, CDC Director Rochelle Walensky, MD, MPH, said there is evidence that the BA.2 sublineage of the Omicron variant is even more infectious but not more severe. “As we’re learning from our colleagues in Europe, Asia, and South Africa, where BA.2 has resulted in varied peaks of cases, there’s no evidence that BA.2 variant results in more severe disease, nor does it appear to be more likely to evade our immune protection,” advised at a recent White House briefing. “However, it does have increased transmission in comparison to the related BA.1 Omicron variant that circulated in the U.S. this past winter.”

Recently, the CDC said that BA.2 has become the dominant version of COVID-19 in the U.S., accounting for nearly 55% of cases that have undergone genetic sequencing.

To reach its conclusions about the earlier version of the Omicron variant, the study team analyzed the electronic health records of more than 651,640 children in the United States who had medical encounters with healthcare organizations between September 2021 and January 2022; those included more than 22,772 children infected with Omicron in late December and late January, who were compared to more than 66,000 children infected when Delta was prevalent in the fall.

The research also included records of more than 10,000 children immediately before the detection of Omicron in the U.S., but when Delta was still predominant. One conclusion was that children infected with Omicron tended to be younger – an average of 1.5 years of age vs. 1.7 years–and had fewer comorbidities.

The age group was selected for study because children younger than 5 are not yet eligible for COVID-19 vaccines and, according to the report, also have a low rate of previous SARS-CoV-2 infections, which also limits their pre-existing immunity.

The study team focused on a 14-day window following SARS-CoV-2 infection, documenting emergency department visits, hospitalizations, ICU admissions and mechanical ventilation use.

“The major conclusion to our research was that many more children were infected with Omicron when compared to Delta, but the children who are infected are not impacted as severely as were children infected with the Delta variant,” said Pamela Davis, MD, PhD, the Arline H. and Curtis F. Garvin Research Professor at the Case Western Reserve School of Medicine. “However, because there are so many more children infected, our hospitals were affected over the winter months by an influx of young children.”

Children were divided into 3 cohorts – the Omicron group, who contracted SARS-CoV-2 infection between Dec. 26, 2021, and Jan.25, 2022, the Delta (B.1.617.2) group who became infected with SARS-CoV-2 between Sept. 1, 2021, and Nov. 15, 2021, and the Delta2 group, who got COVDI-19 between Nov. 16 and Nov. 30, 2021. The researchers developed the Delta2 cohort to control for later time periods and shorter infection windows.

Researchers found that 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 that jumped up to 2.4 to 5.6 cases per 1000 persons per day in December 2021 when the Omicron variant emerged. The study notes that the 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.

Results indicate that the incidence rate of Omicron infection was higher in children aged 0 to 2 years than in those aged 3 to 4 years. “Risks for severe clinical outcomes in children infected with Omicron variant were significantly lower than those in the matched Delta cohort, whereas the risks for severe clinical outcomes in Delta2 cohort did not differ from those in Delta cohort,” the authors report. “There were fewer than 10 deaths in all cohorts.”

“We saw the number of hospitalizations within this age group skyrocket in January of this year because the infection rate of Omicron is about 10 to 15 times compared to that of the Delta variant,” added Rong Xu, PhD, professor of biomedical informatics and director of the Center for AI in Drug Discovery at Case Western’s school of medicine. “Omicron is less severe than Delta, however, the reduction of the severity range in clinical outcomes is only 16 to 85%. Furthermore, since so many unvaccinated children were infected, the long-term effects of COVID-19 infections on the brain, heart, immune systems and other organs of children remains unknown and worrisome. ”

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