Doctors are reporting an increase in the number of children and adolescents with MIS-C, an inflammatory syndrome related to the novel coronavirus—and they're struggling to understand why the condition is surging even as U.S. coronavirus cases are falling steeply.

Cases of rare, coronavirus-linked inflammatory syndrome are on the rise, doctors say

The condition, known as multi-system inflammatory syndrome (MIS-C), is a rare but severe disease that arises in some children and adolescents infected with the novel coronavirus, including some who didn't know they had been infected. The condition typically manifests around two to four weeks after infection, according to CDC. MIS-C shares symptoms with toxic shock and Kawasaki disease, including fever, rashes, swollen glands, and heart inflammation.

In recent weeks, doctors at children's hospitals throughout the United States have said they've noticed an increasing number of child and adolescent patients with MIS-C. According to Axios, the reported spike in MIS-C cases began in January and has continued since, even as the United States overall has seen recent drops in rates of newly reported coronavirus cases, deaths, and hospitalizations.

"As the population in general seems to have fewer active [coronavirus] cases, we are seeing more children being admitted with [Covid-19]-related problems, but most of those—I'd say more than half over the last five weeks—are children who have MIS-C," Rob McGregor, CMO at Akron Children's Hospital, told Axios.

In addition, doctors have said children with MIS-C now appear sicker when compared with children who had MIS-C during earlier parts of America's coronavirus epidemic.

Roberta DeBiasi—chief of infectious diseases at Children's National Hospital in Washington, D.C.—told the New York Times, "We're now getting more of these MIS-C kids, but this time, it just seems that a higher percentage of them are really critically ill."

"The MIS-C has really hit us this time, and the last month has been way higher numbers and higher acuity than we (had) before with MIS-C—and that's hard to explain," Lara Shekerdemian, chief of critical care at Texas Children's Hospital, told Axios.

Shekerdemian added that her hospital also has seen an increase in severe Covid-19 cases. "It feels like ... we have seen in the last two, three months patients who are sicker when they present with [Covid-19] than we did in the early experience," Shekerdemian said.

Although MIS-C cases are climbing nationally, Axios reports that the latest CDC data on the condition—which contains complete reports of MIS-C case numbers through mid-December 2020—shows the syndrome was rare at that point. According to the data, U.S. officials had reported 2,060 cases and 30 deaths related to MIS-C in 48 states, Puerto Rico, and Washington, D.C., by that time.

Most reported cases had occurred among children ranging between the ages of one and 14, but a few cases had occurred among children younger than one and in young adults as old as 20, the data showed. The median age of patients with reported cases of MISC-C was nine, according to the data.

Most of the patients who had reported cases of MIS-C were children of color, with 69% of cases occurring among Hispanic, Latino, or Black children, the data showed.

What's driving the spike?

Experts say it's unclear what's driving the sudden increase in MIS-C cases. However, they noted that the spike began when the United States saw a surge in new coronavirus cases during the winter, which may have increased children's chances of contracting the virus and developing MIS-C.

Doctors have said they expect the MIS-C cases to fall in the coming weeks, in line with the country's recent decrease in coronavirus cases.

"It seems like the peaks we had in the children's hospital lagged a little behind those we were seeing in the adult systems," said Ronald Ford, CMO at Joe DiMaggio Children's Hospital. "I would expect [pediatrics] admissions to start to fall. Now, the big unknown here for everyone is how these new [coronavirus] variants are going to affect things."

Ford noted that it's currently unclear how emerging and more-transmissible variants of the novel coronavirus affect children. "We don't know [how the variants affect children], but that's one of those things that will have to be studied and looked into, if different variants have a different rate of severity of MIS-C in children," he said (Owens/McGhee, Axios, 2/24; Choi, The Hill, 2/16; Bobrowsky, San Francisco Chronicle, 2/20; Belluck, New York Times, 2/24).