**3.2 Epidemiology of MIS-C**

In April 2020, the United Kingdom reported a series of cases with clinical presentation similar to Kawasaki disease (KD), toxic shock syndrome (TSS) and hyper-inflammatory state that had an epidemiological link with SARS-CoV-2 [25]; since that event, clinically similar cases have been reported in other parts of the world, including France, Switzerland, the United States, Canada, Norway, among others [18, 26–28]. After the notification of these cases, an expert consensus among critical care, infectiology, rheumatology and hematology pediatric subspecialists named this new clinical condition "Multisystemic inflammatory syndrome in children".

The worldwide incidence of SARS-CoV-2 in children under 18 years of age is 322 cases per 100,000 inhabitants and the incidence of MIS-C is 2 per 100,000 inhabitants [29]. The first cases were reported in the United Kingdom, as well as in other places in Europe (France, Germany, Greece, Italy, Luxembourg, Portugal, Spain, Switzerland, Sweden), later in Canada and the United States [30]. Most cases of MIS-C occur in previously healthy children older than 8 years and adolescents. Children of African-American and Latino ancestry are the most affected, in contrast to classic KD, which typically affects children under 5 years of age and has a higher incidence in East Asia and in children of Asian descent [31].

The first report of MIS-C was a series of 8 cases receiving medical assistance in southeast England [25]. Subsequently, 3 series of cases were reported in England (n = 58), France and Switzerland (n = 35) and New York (n = 33). In most cases, the children were previously healthy, that is, without underlying comorbidities (88% in the United Kingdom, 89% in France, and 79% in the New York series) [12, 18]. In those with comorbidities, obesity and asthma were the most frequent. The average age was 10 years with an age range of 1 to 17 years [32].

To date, the prognostic factors of severe disease in children are not known, however, a French prospective study that took data from 397 children admitted for COVID-19 in 60 hospitals identified three factors that were independently associated with severe evolution of the disease in the univariate analysis: age ≥ 10 years (OR: 3.4; p = 0.034), hypoxemia (OR: 8.9; p = 0.0004) and C-reactive protein ≥80 mg / L (OR: 6; p = 0.012) [33]. Meanwhile, research presented at the 2021

ENDO Virtual Congress revealed that children with type 1 diabetes whose glycated hemoglobin (A1c) is greater than 9% have a higher risk of severe forms of COVID-19 [34]. At the moment many efforts are being carried out in order to better characterize the pediatric population at risk, with the aim of identifying susceptible populations early and preventing life-threatening events in infants.

#### **3.3 The epidemiology of MIS-C in the Americas**

As of January 14, 2021, a total of 17 countries in the Region of the Americas have officially notified PAHO / WHO or have published information through an official website a total of 2,737 cumulative confirmed cases of MIS-C that coincide chronologically with COVID-19, including 78 deaths [18]. Of the total reported cases, 66% were between 0 and 9 years old at the time of illness and only 10% were in the age group between 15 and 19 years. Regarding the outcome of these cases, the highest proportion of deaths is observed in the age group of 15 to 19 years. Regarding the distribution by sex, 56% of the cases are male [35].

The countries with the highest number of confirmed cases are the United States with 1,659 cases, Brazil with 631 cases, Chile with 151 cases, the Dominican Republic with 102 cases, and Argentina with 65 cases [36]. So far in Colombia, 3 cases of MIS-C have been identified in the district of Cartagena. These cases were detected through media monitoring. The incidence rate of COVID-19 in people under 18 years of age per 100,000 inhabitants in Colombia by department, shows us that the departments and districts above the 75th percentile are: Amazonas, Barranquilla, Atlántico, Bogotá, Cartagena, Chocó, Cesar and Nariño; Between the 50 to 75 percentiles are the departments of Valle del Cauca, Cundinamarca, Santa Marta, Sucre, Tolima, Bolívar, Magdalena, Antioquia. At the 25th percentile are Risaralda, Arauca, Cauca, Santander, Córdoba, Quindío, Caldas and Norte de Santander and below the 25th percentile are Boyacá, Guajira, Meta and Huila [37].

### **3.4 Pathophysiology of MIS-C**

MIS-C is a clinically severe event that mimics other pathologies that present with hyper-inflammatory status in the pediatric population, in mention: KD, SST, Hemophagocytic Lymphohistocytosis (HHL), Macrophage Activation Syndrome (SAM), among others [38]. It is characterized by persistent fever (≥38°C) for more than 24 hours, with involvement of vital organs and consequent cardiological, renal, gastrointestinal, respiratory and / or hematological affection. Patients may present with maculopapular rash, arthritis, and aseptic bilateral conjunctivitis, similar to KD [39].

Symptoms begin 2 to 6 weeks after the resolution of COVID-19 symptoms (in those symptomatic), so it is suggested that it is not due to an effect of the acute event, but to an event mediated by the mechanisms of acquired immunity (cellular and / or humoral) [40]. In the initial stage, fever is usually documented, accompanied by constitutional symptoms, intense headache, general malaise, irritability, GI manifestations such as abdominal pain, vomiting or diarrhea, palmar and / or plantar erythema, mucosal edema, among others less frequent [41].

A range of cardiac dysfunctions are commonly seen with MIS-C, including but not limited to, myocarditis, pericarditis, aneurysms or dilatation of the coronary arteries, valvular insufficiency [1, 16], heart failure [16, 24] and electrocardiographic abnormalities. Other common findings include elevation of troponin, proBNP, C-reactive protein, ferritin, IL-6, D-dimer and need for intensive care [12, 16]. The presence of pericarditis, coronary aneurysms and myocarditis suggest that patients with COVID-19 could have an incomplete form of the Kawasaki disease.

Immunomodulators used in treatment have yielded positive results restoring normal left ventricular function [1] in echocardiographic reports after six weeks of treatment.

Subsequently, the patient presents a hyper-inflammatory state, characterized by an increase in pro-inflammatory cytokines such as: Interleukin 1 (IL-1), Tumor Necrosis Factor alpha (TNF-α), Interleukin 11 (IL-11), Interleukin 12 (IL-12), and especially, Interleukin 6 (IL-6) [42]. Proinflammatory cytokines exert a pleiotropic and redundant effect, which favors the elevation of acute phase reactants and products of the coagulation system, processes termed "immunothrombosis and thromboinflammation" [43]. Lactate Dehydrogenase (LDH), Procalcitonin (PCT), Globular Sedimentation Rate (ESR), C-Reactive Protein (CRP), Serum Ferritin, Serum Amyloid A, Fibrinogen, and D-dimer among other biomarkers may elevated. This state can produce functional alterations at the endothelial level, generating an imbalance between the homeostatic mechanisms of vasoactive control, leading to a state of severe hypotension and the consequent cardiogenic shock. These conditions may lead to multiple organ failure and death in some cases [44].

Immunologically, the mechanisms underlying the hyperinflammation state are not known; however, there are some findings that suggest certain molecular and cellular mechanisms. Regarding immunogenetics (Major Histocompatibility Complex, MHC, and HLA molecules), Nguyen and Cols, by immunoinformatic analysis, examined how HLA variation could affect the cellular immune response against coronavirus peptides that infect humans [45]. The researchers found that the HLA-B \* 46: 01 allele has few SARS-CoV-2 peptide binding sites, while the HLA-B \* 15: 03 allele showed greater ability to recognize and display highly conserved peptides in SARS-CoV-2, which suggests that host genetic factors may play a role in cellular immune response and clinical presentation in response to SARS-CoV-2 infection [46].

The superantigen hypothesis has also been proposed to understand and clarify the immunological events that support MIS-C [47]. This hypothesis suggests that the SARS-CoV-2 virus produces super antigens that activate the immune system. A superantigen refers to peptides (sometimes motifs and / or proteins) that bind to T lymphocytes of an individual, expressing a particular group or family of genes on the β chain of the variable region (Vβ) of the T-cell receptor (TCR) [48]. The binding of the superantigen with the Vβ domains of the TCR leads to their polyclonal activation, leading to the production of large amounts of cytokines and a clinical syndrome similar to septic shock, similar to what occurs in MIS-C. Superantigens are presented to T-cells through binding to non-polymorphic regions of HLA-II molecules located on antigen presenting cells (APC) and interact with conserved regions of the Vβ domains of the TCR. For example, several staphylococcal enterotoxins are SAg [49].

By structure-based computational modeling Rivas and Cols discovered that the SARS-CoV-2 Spike (S) protein possesses a high affinity motif located close to the S1 / S2 cleavage site with a highly conserved sequence to superantigens [50]. The region containing this motif exhibits a high binding affinity to the complementarity determining regions (CDRs) present in the variable domains of the α and β chain of the TCR. This region is highly similar to the primary sequence and threedimensional structure of a superantigen fragment corresponding to staphylococcal enterotoxin B (SEB), which interacts with the TCR and CD28 of T cells [51].

Next-generation immuno-sequencing of the TCR repertoire of COVID-19 patients indicated that the severity of the infection may be associated with some genes that encode the Vβ region of the TCR [52]. Using structure-based computational modeling, Cheng and cols. Demonstrated that the SARS-CoV-2 protein S exhibits a high-affinity TCR-binding motif, being able to form a complex with HLA-II molecules. The researchers argue that this interaction between the virus and human T cells could be enhanced by a rare mutation (D839Y / N / E) from a European strain of SARS-CoV-2 [52]. The studies also found that the SARS-CoV-2 protein S possesses a neurotoxin-like sequence motif in the receptor-binding domain, which exhibits a high tendency to bind to TCR. These findings are consistent with the clinical presentation of patients with MIS-C, who exhibit hyperinflammation and neurological symptoms suggestive of neurotoxicity [53, 54].
