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Remember MIS-C, the “multisystem inflammatory syndrome in children” that was all the rage during the early months of the SARS-CoV-2 pandemic? I certainly do. Children with MIS-C, in addition to fever, present with non-specific complaints such as abdominal pain, vomiting, diarrhea, rash, and conjunctivitis (pink eye) a few weeks after COVID-19, which could have been a mild or even undetected case. These patients then go on to develop an intense inflammatory response throughout the body that can injure multiple organs, in particular the heart, leading to shock and the need for intensive care.

I have seen just a couple cases of MIS-C so far, both in the first year of the pandemic and both involving an extremely ill child who needed transfer to a facility with a pediatric intensive care unit. The possibility of a patient developing this dreaded complications of COVID-19 had many of worried every time an ill child came into the hospital with unexplained fever. This was a completely new and quite serious clinical entity and not much was known about the typical course in those early months. Nobody wanted to delay diagnosis and appropriate medical care.

This meant that work-ups for kids with fever became more aggressive than usual for a while. Though uncommon even at its peak, fear of MIS-C and some (necessarily) hastily thrown together diagnostic algorithms meant that a lot of children underwent expanded laboratory evaluations who wouldn’t have in the past. And this meant that a lot of children were admitted for close observation for 24 hours when some of these labs were abnormal. It was, I admit, a bit of a mess for a while. We did our best.

Most of that anxiety has cooled, not that MIS-C has gone away. We have become more comfortable with the clinical assessment but cases have also plummeted. Throughout the pandemic, spikes in the incidence of MIS-C have followed spikes in cases of COVID-19, and about 1 out of every 3,000 to 4,000 children with COVID-19 developed MIS-C during the worst months of 2020 and 2021. Thankfully the risk has decreased dramatically since the early days, likely because of genetic changes in subsequent dominant variants as well as the positive impact of vaccines and protective antibodies acquired from previous infection.

Earlier this month, a report from the CDC highlighted the importance of COVID-19 vaccination in preventing MIS-C. Researchers looked at every case of MIS-C reported to the CDC surveillance system through the end of February that had occurred in 2023, which amounted to 117 total cases. The total number of cases in reality was likely a bit higher, however, because not all jurisdictions have reported their 2023 numbers yet. There was also a change in the case definition of MIS-C for surveillance purposes last year which may have caused some confusion and an undercount of true cases.

Even with this potential limitation, there were some interesting findings. The overall incidence was much lower (0.11 cases/million person-months) compared to 2022 (0.56 cases/million person-months), and far below the peak of 6.79 cases/million person-months during October 2020 through April 2021. The median age of patients diagnosed with MIS-C in 2023 also shifted younger, decreasing from 9 years during the early days down to 7 years. This is still older than the typical age when Kawasaki disease, an inflammatory vasculitis that has significant clinical overlap with MIS-C, is diagnosed.

Half of the patients required intensive care, with about a third developing shock and a quarter developing cardiac dysfunction. This is only a little better compared with severity seen in the early days of the pandemic, meaning that while MIS-C incidence is much lower it remains every bit as serious a condition. The mortality rate in the 2023 cases was 3%.

When it comes to the relationship between MIS-C and vaccine status, the results are impressive. 112 (96%) of patients with MIS-C were eligible for the vaccine but only 20 had received even one dose. And for 12 of those 20 it had been more than a year since their last dose. That means 93% were either unvaccinated or far out from their last vaccination, lending support for yearly boosters that are recommended by the CDC.

Being current with the COVID-19 vaccine dramatically reduces the risk of developing MIS-C. It may not be a common problem, but it is a bad one and kids deserve to be protected. Add this to the fact that these vaccines are incredibly safe and also reduce the risk of severe acute COVID-19 and it sure seems pretty clear that it is the right thing to do.

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  • Clay Jones, M.D. is a pediatrician and a regular contributor to the Science-Based Medicine blog. He primarily cares for healthy newborns and hospitalized children, and devotes his full time to educating pediatric residents and medical students. Dr. Jones first became aware of and interested in the incursion of pseudoscience into his chosen profession while completing his pediatric residency at Vanderbilt Children’s Hospital a decade ago. He has since focused his efforts on teaching the application of critical thinking and scientific skepticism to the practice of pediatric medicine. Dr. Jones has no conflicts of interest to disclose and no ties to the pharmaceutical industry. He can be found on Twitter as @SBMPediatrics and is the co-host of The Prism Podcast with fellow SBM contributor Grant Ritchey. The comments expressed by Dr. Jones are his own and do not represent the views or opinions of Newton-Wellesley Hospital or its administration.

Posted by Clay Jones

Clay Jones, M.D. is a pediatrician and a regular contributor to the Science-Based Medicine blog. He primarily cares for healthy newborns and hospitalized children, and devotes his full time to educating pediatric residents and medical students. Dr. Jones first became aware of and interested in the incursion of pseudoscience into his chosen profession while completing his pediatric residency at Vanderbilt Children’s Hospital a decade ago. He has since focused his efforts on teaching the application of critical thinking and scientific skepticism to the practice of pediatric medicine. Dr. Jones has no conflicts of interest to disclose and no ties to the pharmaceutical industry. He can be found on Twitter as @SBMPediatrics and is the co-host of The Prism Podcast with fellow SBM contributor Grant Ritchey. The comments expressed by Dr. Jones are his own and do not represent the views or opinions of Newton-Wellesley Hospital or its administration.