Patient 1 was a 15-year-old girl with asthma who received her first dose of BNT162b2 6 days before seeking care. She had low-grade fever and myalgia, which resolved within 2 days of vaccination. Four days later, she experienced 102°F fevers, headaches, nonbilious emesis, myalgias, chest pain, and a rash. Emergency department (ED) examination identified pharyngeal erythema, bilateral conjunctivitis, and a diffuse blanching rash. She had no respiratory or cardiovascular symptoms. At admission, laboratory test results showed leukocytosis with polymorphonuclear cell predominance and elevated CRP, fibrinogen, prothrombin time, brain natriuretic peptide (BNP), and D-dimer (Table). Urinalysis revealed trace protein, large blood, moderate leukocyte esterase, 10–20 leukocytes per high-powered field, and 1+ bacteria. Results of nasopharyngeal SARS-CoV-2 reverse transcription PCR were negative. Further tests included chest radiograph, chest computed tomography angiography, electrocardiogram, and echocardiogram; all results were unremarkable. She was admitted to the pediatric intensive care unit (ICU) and given 2 g/kg intravenous immune globulin (IVIG) for suspected of MIS-C. Symptoms rapidly improved. Leukocyte level decreased to 11.0 K/uL and D-dimer to 2.5 mg/L within 48 hours. The patient remained hemodynamically stable throughout admission and was afebrile with improved symptoms when she was discharged 3 days after admission. SARS-CoV-2 antibody test results at discharge were positive for nucleocapsid but negative for spike. Two days after discharge, the patient returned to the ED for throbbing headaches, nausea, and fatigue. CRP had downtrended since discharge to 2.71 mg/L. Magnetic resonance venography results were normal and she was discharged on antimigraine medication.

Patient 2 was a previously healthy female 17-year-old who received her first dose of BNT162b2 vaccination 7 days before seeking care. Three days after vaccination, she experienced fevers, headaches, abdominal pain, fatigue, and myalgias. Her primary care provider noted leukocytosis to 20 K/uL and referred her to the ED. She had a 103.1°F fever, diffuse abdominal tenderness, and costovertebral angle tenderness. She had no respiratory symptoms. At admission, laboratory test results showed leukocytosis with polymorphonuclear cell predominance and elevated CRP, erythrocyte sedimentation rate, lactate dehydrogenase, BNP, troponin, D-dimer, creatinine, aspartate aminotransferase, and alkaline phosphate (Table). Urinalysis revealed 100 mg/dL protein, moderate blood, moderate leukocyte esterase, 10–20 leukocytes per high-powered field, 5–10 red blood cells per high powered field, and no bacteria. Urine culture was positive for 10,000 CFU/mL of Escherichia coli. Blood culture results were negative. Electrocardiogram showed sinus tachycardia and nonspecific T-wave abnormalities. Abdomen and pelvis computed tomography showed diffuse left renal enlargement without hypoattenuation or hyperattenuation and possible polycystic ovaries. Results of chest radiograph and echocardiogram were normal. Nasopharyngeal SARS-CoV-2 RT-PCR was negative. Results of SARS-CoV-2 spike antibody testing were positive; nucleocapsid antibody testing was not performed. She started 3 days of intravenous methylprednisolone (30 mg 2×/d) and 1 day IVIG (2 g/kg) for MIS-C. Troponin decreased to <0.05 within 24 hours and CRP to 16.2 within 48 hours. BNP rose to 2,024 on hospital day 2. Repeat echocardiogram showed mild right coronary artery ectasia, and she was started on 325 mg of aspirin daily. On hospital day 3, repeat echocardiogram results were normal, and she was afebrile. Aspirin was decreased to 81 mg daily. She was discharged on hospital day 4 with no fevers for 60 hours and downtrending inflammatory markers including CRP to 8.49 mg/dL. She was also treated for a possible UTI.

This report describes 2 cases of MIS-C within 1 week of receiving the first dose of BNT162b2. There is no specific test for MIS-C; although both patients met diagnostic criteria, alternative diagnoses were possible. Patient 2 had costovertebral angle tenderness, unilateral renal enlargement, and 10,000 CFU/mL growth of a uropathogen on culture. Given the low level of bacterial growth, lack of enhancement on her CT, and constellation of lab and imaging abnormalities not commonly seen with urinary tract infections, MIS-C remains her most likely diagnosis.

Patient 1 had a positive antinucleocapsid antibody suggesting community-acquired COVID-19 infection before MIS-C developed (P.D. Burbelo et al., unpub. data, https://doi.org/10.1101/2020.04.20.20071423). Salzman et al. describe 3 similar cases in which MIS or an MIS-like illness developed after COVID-19 vaccination, particularly in the setting of community-acquired COVID-19 (3). The chronology of events in these cases raises the possibility that vaccination may be involved in the pathogenesis of MIS-C when preceded by community-acquired SARS-CoV-2.

The pathogenesis of MIS-C is thought to involve immune dysregulation and hyperinflammation (4). Studies have identified high levels of receptor-binding protein (RBD) antibodies in children with severe MIS-C (5,6). Both natural SARS-CoV-2 infection and BNT162b2 vaccination have been shown to elicit RBD antibodies (7). It may be possible that the immune responses to these 2 forms of exposure to SARS-CoV-2 interact to shape the manifestations of mild MIS-C in the postinfectious period of COVID-19. Although both of these cases were mild, we have insufficient data on the pathogenesis of MIS-C to understand how vaccination may shape symptomatology.

A recent report by Zambrano et al. documented that 61/97 (62.9%) MIS-C cases in unvaccinated patients required ICU admission (8). That report had a small number of vaccinated cases; 1 in 5 of those vaccinated needed ICU care (8). An analysis of postvaccination MIS-C in 21 patients showed that 3 (14%) required invasive mechanical ventilation, 8 (38%) required vasopressors, and 12 (57%) required ICU care (A.R. Yousaf et al., unpub. data, https://doi.org/10.1101/2022.01.03.22268681). In contrast to Zambrano et al.’s vaccinated cases and our reported cases, the Yousaf et al. report suggests a similar number of ICU admissions in vaccinated and unvaccinated persons.

Studies have shown that COVID-19 vaccination is associated with reduced incidence of MIS-C, especially if 2 doses are given. A study of MIS-C cases in France during September–October 2021 found a significantly lower risk of MIS-C among vaccinated adolescents than those who were unvaccinated (9). Zambrano et al. found a 91% protective effect of complete (2 doses) BNT162b2 vaccination against MIS-C (8). Phase 2 and phase 3 clinical trials of BNT162b2 revealed 0 cases of MIS-C after vaccination (10). Despite the reports of postvaccination MIS-C, vaccination clearly lowers the overall MIS-C burden, probably by preventing infection. These studies also suggest low likelihood of vaccination triggering development of MIS-C.

If vaccination can play a role in MIS-C pathogenesis, it is likely an extremely rare event and may involve an underlying genetic predisposition or be contingent on extraneous factors like recent SARS-CoV-2 community exposure. Our findings in 2 cases of MIS-C within 1 week of a dose of BNT162b2 raise the possibility that vaccination may alter the symptom profile of MIS-C.