Surveillance for multisystem inflammatory syndrome in children — United States, 2023

March 15, 2024
Notes from the field.

MIS-C incidence has decreased from early in the COVID-19 pandemic (highest in late 2020–early 2021), but cases continue to occur with a recent relative increase in the fall of 2023 after a period of increased COVID-19 activity in the general population, according to a MMWR report.

Among 117 patients with MIS-C in 2023, approximately one half required intensive care unit–level care. More than 80% (92 of 112) of MIS-C cases were in vaccine-eligible but unvaccinated children, and among the 20 vaccinated children, 60% likely had waned immunity at the time of MIS-C illness.

All MIS-C cases reported to CDC national surveillance as of February 26, 2024, with illness onset during 2023 were included, and patient characteristics were analyzed. Incidence (cases per 1,000,000 person-months) was estimated using bridged-race 2020 population estimates from U.S. Census Bureau data. COVID-19 vaccination status was reported for children who were age-eligible for vaccination at the time of MIS-C illness onset. This activity was reviewed by CDC, deemed not research, and was conducted consistent with applicable federal law and CDC policy.

Among 117 MIS-C patients with illness onset in 2023, 31 (26%) had onset during August–October, after an increase in COVID-19 activity earlier in the summer; this finding represented a two-thirds increase in case counts compared with the 19 (16%) cases reported with onset during the preceding 3 months. Overall MIS-C incidence in 2023 was 0.11 cases per million person-months (95% CI = 0.10–0.14), representing an 80% decline in incidence compared with that during April–December 2022 (0.56 cases per million person-months; 95% CI = 0.51–0.62), and a 98% decrease from the peak of 6.79 (95% CI = 6.56–7.03) early in the COVID-19 pandemic (October 2020–April 2021).

The median age of MIS-C patients with illness onset in 2023 was 7 years, whereas the median age during February 2020–January 2022 was 9 years, and during April–December 2022 was 5 years. A similar decline in MIS-C incidence and shift to a younger age group in 2022 was reported in England.

Among the 117 MIS-C patients with illness onset in 2023, 68 (58%) had no underlying medical conditions; 58 (50%) required intensive care unit (ICU)-level care, 40 (34%) experienced shock, and 31 (27%) experienced cardiac dysfunction. These prevalences are similar to published national MIS-C surveillance data for 2,116 cases reported during July 9, 2021–January 31, 2022 (52% requiring ICU-level care, 38% with shock, and 29% with cardiac dysfunction), and are improved compared with data for cases reported for the total 4,470 cases during the earliest part of the pandemic, from February 19, 2020–July 31, 2021 (63% requiring ICU-level care, 45% with shock, and 31% with cardiac dysfunction).

Three (3%) patients with MIS-C died in 2023. Although 112 (96%) patients were age-eligible for COVID-19 vaccination, only 20 (18%) had documented receipt of any COVID-19 vaccine. Among the 48 vaccine-eligible patients with underlying medical conditions, nine (19%) had documented receipt of any COVID-19 vaccine. Among the 20 patients who had received COVID-19 vaccination, 12 (60%) received their last dose >12 months before MIS-C onset.

Read the full report at the CDC