Kids' COVID syndrome—MIS-C—less severe in Omicron

COVID-19–related multisystem inflammatory syndrome in children (MIS-C) was milder amid the Omicron variant surge than during the Alpha and Delta waves in Israel, concludes a research letter published late last week in JAMA.

Researchers from Rambam Health Care Campus in Haifa conducted prospectively studied all pediatric MIS-C patients at 12 Israeli hospitals during the same 16-week period in the Alpha (Dec 20, 2020, to Apr 10, 2021), Delta (Jul 18 to Nov 13, 2021), and Omicron (Nov 21, 2021, to Mar 12, 2022) pandemic waves. Participating hospitals account for roughly 70% of pediatric intensive care unit (ICU) admissions in Israel.

Of the 171 MIS-C patients, 59 (34.5%) were diagnosed during the Alpha wave, while 79 (46.2%) were identified during Delta, and 33 (19.3%) amid Omicron. Median patient age was 8 years, and 55% were boys.

Five of 79 patients (6.3%) in the Delta wave and 5 of 33 (15.1%) during Omicron had received a second COVID-19 vaccine dose at least 2 weeks before hospitalization. (Vaccination wasn't widely available in the Alpha wave.) No vaccinated patients required ICU care or treatment with vasopressors to raise their blood pressure.

Better cardiac outcomes, fewer ICU stays

Cardiac outcomes were better during the Omicron wave, and there were fewer ICU admissions (7 [21.2%]) than during Delta (39 [49.4%]) and Alpha (34 [57.6%]). Median hospital stay was also 2 days shorter amid Omicron than in previous waves.

Fewer patients required vasopressors during the Omicron surge (6.0%) than in Alpha (22%) and Delta (17.7%). Likewise, no patients needed mechanical ventilation amid Omicron, versus 8.5% in the Alpha period and 8.9% in Delta. One patient died in the Delta era.

Nationwide, 188,800 pediatric patients were diagnosed as having COVID-19, and 103 had MIS-C during the Alpha wave, while 233,585 had COVID-19 and 115 had MIS-C amid Delta, and 945,779 and 36 had COVID-19 and MIS-C, respectively, during Omicron.

The incidence of MIS-C per 100,000 children was 54.5 during the Alpha wave, 49.2 amid Delta, and 3.8 in the Omicron era. Compared with the Omicron period, the incidence of MIS-C was 14.3 times higher amid Alpha and 12.9 times higher during Delta.

The lesser severity and incidence of MIS-C during the Omicron wave relative to the Alpha and Delta periods may be explained by the nature of the Omicron variant itself, previous COVID-19 infections, vaccination, and the refinement of treatment methods since the pandemic began, the study authors said.

"A 2022 study from South Africa on the Omicron wave reported no cases of MIS-C, a finding that corroborates these results," they wrote.

Study limitations included the small sample size and single-country data, the researchers said. "Because MIS-C is a late-onset phenomenon of SARS-CoV-2 infection, cases that appeared after the 16-week period of each wave were not included," they concluded.

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