Patient is a 13 year old 10 month old previously healthy male who presented on 8/16/21 w/ fever, nausea, headache and body aches x2d and vomiting, diarrhea, and altered mental status x1d secondary to MIS-C vs acute vaccine reaction in the setting of recent 2nd dose of Pfizer COVID-19 vaccine (8/14/21), admitted to the peds wards at MC on 8/16/2021 12:21 PM for IVIG, steroids, cardiac work-up, and supportive care.    Patient received his second dose of the Pfizer COVID-19 vaccine on 8/14/21 and began to develop headache, fever, body aches, and nausea that night. The next morning he continued to have intermittent fevers defervesced by Tylenol but developed NBNB emesis x1 w/PO intake and multiple loose stools. Per dad, by Sunday evening patient had unsteady gait and AMS, seemingly talking to himself and not forming intelligible sentences. On Monday morning patient continued to have NBNB emesis x2 w/Pedialyte intake, fevers, and chills and presented to the ED. Pt denied recent trauma, LOC, seizures, change in vision/hearing, photo/phonophobia, runny nose, ear pain, sore throat, cough, neck pain, SOB, diaphoresis, chest pain, palpitations, hematemesis, melena, bloody stools. No extremity pain or swelling. No rash. No recent illness, travel, or sick contacts. Dad, with whom patient lives, is vaxxed against COVID-19 and they have no known COVID-19 contacts. Patient does not recall viral URI or cold sx in the past few weeks, although cites possibility of runny nose and sore throat sometime in July which self-resolved.   ED course included labs significant for elevated trop (398), ESR (14), CRP (108.3), low Na (128), elevated transaminases (AST 83, ALT 85), Tbili (1.7), coags (APTT 42, INR 1.5), d-dimer (1.58), triglycerides (214), and CK (2,261). A CXR and EKG were unremarkable. US Abd was significant for "starry sky" appearance of the liver. Given his presentation and lab findings concerning for potential MIS-C, patient was transferred to MC for supportive care and additional work-up. Additional labs were ordered and in the ED he received 3 boluses, Zofran 4mg, Tylenol 857.8mg, Ibuprofen 400mg, and IVF were started.   Given unremitting fevers, suggestive clinical features (GI sx, neuro sx, conjunctivitis), and significant lab findings (elevated CRP, ESR, hyponatremia, elevated trop and CK, elevated coags, elevated triglycerides) assessment on admission was likely MIS-C 2/2 COVID-19 vaccine or prior asymptomatic COVID-19 infection vs acute vaccine reaction. Pt spiked a fever a few hours after admission w/chills and rigor and was started on MIS-C treatment: 125g IVIG (2g/kg) (8/16-8/17), 30mg Solumedrol BID, GI ppx,Vitamin K 5mg QD x3d (8/16-8/18), Ofirmev 650mg/dose Q6 PRN, and Rocephin 1g/day DAILY until sepsis/infection could be ruled out. By 8/17/21 patient was no longer having fevers and blood culture at 24h demonstrated no growth, Rocephin was discontinued. Labs began to downtrend, including trop, d-dimer, ck, bnp, crp. Repeat EKG was unremarkable and ECHO demonstrated normal study for age w/no reduced LVEF or CAA.    On 8/18/21 patient achieved >48 hrs of no fever and improving labs, with good PO intake and ability to tolerate PO meds, as well as stable vital signs. However, his CBC panel indicated WBCs, HgB, and plts decreasing over the prior two days.    On the morning of 8/19/21 his WBCs, HgB, and plts were rebounding and trop, CRP continued to downtrend. Patient was afebrile, stable, tolerating PO intake and meds well >48h and was discharged home to f/u in 1 week with Cards for activity clearance and f/u ECHO at 1-2 weeks and 4-6 weeks after sx onset. Pt was also to f/u w/ID in 1 week w/repeat labs (CBC, manual differential, CRP, lytes, Albumin, Ddimer, Fibrinogen, Ferritin, AST/ALT , troponin, BNP) to be completed outpatient prior to f/u. Patient to continue outpatient PO steroids and pantoprazole.    Of note, pt's COVID-19 Nucleocapsid Antibody came back negative indicating patient does not have recent COVID-19 infection despite clinical signs/sx and lab findings consistent with MIS-C picture, including: unremitting fevers, GI sx (n/v and loose stool), neuro sx (AMS), conjunctivitis, elevated CRP, ESR, hyponatremia, elevated trop and CK, elevated coags, elevated triglycerides. Etiology of illness perhaps 2/2 acute vaccine rxn however inconsistent with sx peak presentation at 48h post-vax rather than within 24h. Some rare cases of MIS-C following COVID-19 vaccine reported to the CDC, however, in these patients there had also been prior known COVID-19 infection or SARS-CoV-2 PCR and nucleocapsid IgG positive.