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MIS-C cases in 2023 were mostly among unvaccinated children: CDC

Cases of MIS-C were still occurring in 2023 and children who are unvaccinated or have waning immunity from previous vaccination are at the highest risk of developing the condition, according to a new federal report published Thursday.

MIS-C, or multisystem inflammatory syndrome in children, is a rare but serious condition in which different body parts can become inflamed -- such as the heart, lungs, brain and kidneys -- and is often seen in children after they are diagnosed with COVID-19, according to the Centers for Disease Control and Prevention.

It typically occurs between two to six weeks after infection and many children can be hospitalized with serious complications.

MORE: MiS-C patients more likely to have neurologic and mental health effects: Study

To better understand MIS-C, CDC investigators and partners from various health departments looked at cases of MIS-C in 2023 and compared them with cases that occurred earlier in the pandemic, from 2020 to 2022.

Although incidence of MIS-C dropped dramatically, particularly since the peak from late 2020 to early 2021, there continues to be cases and, specifically, an increase during fall 2023, when COVID-19 activity began peaking in the U.S.

PHOTO: Emerson Ngim, of San Jose, gets her Covid-19 vaccination from licensed vocational nurse  Sharon Villegas at the Santa Clara County Fairgrounds in San Jose, June 21, 2022.  (Jane Tyska/The Mercury News/Getty Images, FILE)
PHOTO: Emerson Ngim, of San Jose, gets her Covid-19 vaccination from licensed vocational nurse Sharon Villegas at the Santa Clara County Fairgrounds in San Jose, June 21, 2022. (Jane Tyska/The Mercury News/Getty Images, FILE)

In total, 117 cases were reported during 2023 with more than one-quarter -- 26% -- occurring between August and October 2023. The authors, however, said this is likely an undercount due to reporting lags.

"While numbers have gone down, it's still a persistent risk in our pediatric population and it's especially true following surges of COVID. We will see increases in this condition among our kids," said Dr. John Brownstein, an epidemiologist and chief innovation officer at Boston Children's Hospital and an ABC News contributor.

The median age of MIS-C patients in 2023 was 7. This is younger than the median age of 5 from February 2020 to January 2022.

Of the 117 patients, 58% had no previous underlying conditions. This is consistent with previous data from the CDC, which has shown that majority of children with MIS-C don't have pre-existing conditions.

When children did have pre-existing conditions, obesity was the most common condition at 27.4%. Other pre-existing conditions included chronic lung disease, neurological conditions and cardiac conditions.

MORE: Health officials report uptick in children's rare COVID-related inflammatory syndrome following delta surge

Many children required intensive medical care. The report found 50% were admitted to intensive care units; 34% experienced shock, meaning the organs weren't getting enough blood or oxygen; and 27% experienced cardiac dysfunction. The team found the percentages were similar to those reported earlier in the pandemic.

The overwhelming majority, 112 of the 117 patients, were eligible for COVID-19 vaccines at the time of their illness.

However, 82.1% of the children with MIS-C were unvaccinated. Additionally, among the 20 patients who were vaccinated, 60% were more than 12 months out from their last dose of the vaccine. Only five children with MIS-C had received three or more doses. `

This means that unvaccinated children and those with waning immunity were more likely to develop MIS-C than those who were vaccinated or whose immunity hadn't waned.

PHOTO: A sick child is pictured in this stock image.  (STOCK PHOTO/Kathleen Finlay/Getty Images)
PHOTO: A sick child is pictured in this stock image. (STOCK PHOTO/Kathleen Finlay/Getty Images)

"It highlights the fact that vaccination still represents our critical tool to reducing any significant risk of complications from COVID in our pediatric population," Brownstein said. "The issue, of course, is that as we get further out from the sort of height of the pandemic, vaccination becomes less of a priority for families."

Brownstein said this may be highlighted by the age shift in younger kids contracting MIS-C in 2023 compared to previous points in the pandemic.

"Likely because those kids have become vaccine-eligible, but parents are electing not to vaccinate their kids, because of the sort of overall perception that COVID risk is low," he said. "But MIS-C is one of those critical features of COVID that should represent an important reason why parents should get their kids vaccinated."

MIS-C cases in 2023 were mostly among unvaccinated children: CDC originally appeared on abcnews.go.com