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Cases of multisystem inflammatory syndrome in children (MIS-C) are still popping up, but at lower rates than those observed early in the COVID-19 pandemic, CDC researchers said.

In 2023, MIS-C incidence was 0.11 cases per million person-months, representing an 80% decrease in incidence compared with April-December 2022 (0.56 cases per million person-months) and a 98% decrease from the peak of 6.79 early in the pandemic (October 2020-April 2021), reported Anna Yousaf, MD, of the CDC's National Center for Immunization and Respiratory Diseases, and colleagues.

Among 117 MIS-C patients in 2023, 26% had illness onset from August through October, following an increase in COVID activity earlier in the summer -- a two-thirds increase in cases compared with the 16% of cases reported with onset during the preceding 3 months, they noted .

"Clinicians should recognize that MIS-C might occur, especially during and after periods of increased COVID-19 activity, and should be familiar with treatment guidelines," Yousaf and colleagues wrote. "Continued reporting of MIS-C cases to jurisdictional health departments is important to monitor trends and patients' demographic and clinical characteristics."

Though rare, MIS-C is serious, they said. It typically occurs 2 to 6 weeks after SARS-CoV-2 infection and is characterized by fever and multi-organ involvement.

Of the patients with MIS-C in 2023, 3% died.

Though 96% of patients were age-eligible for COVID vaccination, only 18% had documented receipt of a vaccine. Additionally, among 48 vaccine-eligible patients with underlying medical conditions, just 19% had documented receipt. Of the 20 patients who had been vaccinated, 60% received their last dose more than 12 months prior to MIS-C onset.

"MIS-C patients with illness onset in 2023 were predominantly unvaccinated children and those whose vaccine-induced immunity had likely waned," Yousaf and colleagues wrote. "COVID-19 vaccination remains an important tool for preventing MIS-C."

Of the MIS-C patients in 2023, 58% had no underlying medical conditions, Yousaf and colleagues reported. Half required intensive care unit (ICU)-level care, 34% experienced shock, and 27% experienced cardiac dysfunction.

These figures were similar to national MIS-C surveillance data for 2,116 cases reported from July 9, 2021 to Jan. 31, 2022, and were improved compared with data for 4,470 cases from Feb. 19, 2020 to July 31, 2021. For the latter cases reported during the earliest part of the pandemic, 63% required ICU-level care, 45% experienced shock, and 31% experienced cardiac dysfunction.

The median age of MIS-C patients in 2023 was 7 years compared with a median age of 9 years from February 2020-January 2022, and 5 years from April-December 2022.

For their report, Yousaf and colleagues examined all MIS-C cases reported to CDC national surveillance as of Feb. 26, 2024, with illness onset in 2023. Incidence was estimated using population estimates from U.S. Census Bureau data. COVID vaccination status was reported for kids who were age-eligible for vaccination at the time of MIS-C illness onset.

Yousaf and colleagues acknowledged that 2023 incidence is "likely an underestimate" because jurisdictional reporting of MIS-C cases with illness onset in 2023 was incomplete, and because case counts might have been affected by a change in case definition.

"Changes might also reflect changing SARS-CoV-2 population immunity from vaccination and previous infection," they added, "and characteristics of the predominant circulating SARS-CoV-2 variants."

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