MIS-Children and MIS-Adults were first recognized as a serious sequela to COVID-19. It is a poorly understood, underreported, hyperinflammation of multiple organ systems following COVID infection. It is often not diagnosed in a timely manner. MIS is defined as:
- Individual presenting with fever
- Laboratory evidence of inflammation
- Clinically severe illness with multisystem organ involvement
- AND evidence of recent COVID infection and/or vaccination.
The CDC recently published a report on MIS in children 12-20 following COVID vaccination. https://www.thelancet.com/journals/lanchi/article/PIIS2352-4642(22)00028-1/fulltext
Many reports of MIS following vaccination have begun to emerge as well.
The CDC Published a MIS-Associated with COVID report form: https://stacks.cdc.gov/view/cdc/88583
Going down the checklist, COVID long haulers and the COVID vaccine injured often find themselves meeting the criteria for MIS diagnosis, the exception being the vaccine injured without prior COVID infection. Multiple organ system involvement and laboratory evidence of systemic inflammation are the hallmarks of MIS. MIS has been postulated to involve a dysregulated immune response. Studies are attempting to understand the pathophysiology of MIS and weather endotheliitis, autoimmune and/or other mechanisms are responsible for hyperinflammation. https://jamanetwork.com/journals/jamanetworkopen/fullarticle/2784427 Most patients with MIS presented with fever (96%), hypotension (60%), cardiac dysfunction (54%), shortness of breath (52%) and diarrhea (52%) Affected organ systems are hematologic, cardiovascular, gastrointestinal, and respiratory. Myocarditis was diagnosed in 30% and cardiac dysfunction in 54% of the reviewed MIS cases. Early and appropriate diagnosis of MIS events is essential for definite therapy. Treatment of MIS includes; corticosteroids, intravenous immunoglobulin (IVIG), anticoagulants and immune modulators. Early treatment is essential to reduce tissue damage and improve patient outcome. A high index of suspicion must be kept for MIS-V and COVID-19 vaccine related myocarditis to reduce morbidity and have a favorable outcome. 
Proposed mechanism of MIS, COVID spike protein initiates cascade of events leading to hyperinflammatory syndrome The autopsy of two adolescents who died of myocarditis following the second dose of COVID vaccine https://pubmed.ncbi.nlm.nih.gov/35157759/ has revealed that the type of myocarditis caused by vaccines is consistent with “catecholamine-induced injury, not typical myocarditis pathology” 
The heart damage seen is consistent with MIS-C caused heart inflammation 
The authors reference and discuss the autopsy and biopsy of two adult cases of myocarditis post vaccination. The findings are the same. The injury appears to be due to hyperinflammation rather than typical myocardial injury. 
The same inflammatory mechanisms appear to be at play in both COVID and vaccine induced MIS, indeed, the observed increase in myocarditis and pericarditis post COVID vaccine may be due to the same MIS causing mechanisms. 
Early recognition and treatment is key. Steroids should be employed early and supported with IVIG and additional therapies as required. Clinicians must be aware, diligent and exercise prudence in recognizing clinical and laboratory findings suggestive of MIS diagnosis and be proactive in treating suspected cases. 