indolent systemic mastocytosis; This literature-non-study case was reported in a literature article and describes the occurrence of SYSTEMIC MASTOCYTOSIS (indolent systemic mastocytosis) in a  patient of an unknown age and gender who received mRNA-1273 (Moderna COVID-19 Vaccine) for COVID-19 prophylaxis.        No Medical History information was reported.    On an unknown date, the patient received dose of mRNA-1273 (Moderna COVID-19 Vaccine) (unknown route) 1 dosage form. On an unknown date,  after starting mRNA-1273 (Moderna COVID-19 Vaccine), the patient experienced SYSTEMIC MASTOCYTOSIS (indolent systemic mastocytosis) (seriousness criterion medically significant). At the time of the report, SYSTEMIC MASTOCYTOSIS (indolent systemic mastocytosis) outcome was unknown.      DIAGNOSTIC RESULTS (normal ranges are provided in parenthesis if available): On an unknown date, Biopsy bone marrow: Subsequent bone marrow and diagnostic criteria confirmed indolent systemic mastocytosis.. On an unknown date, Tryptase: 25.7 microgram per litre elevated serum tryptase level obtained weeks after his vaccine (25.7 µg/L).     The action taken with mRNA-1273 (Moderna COVID-19 Vaccine) (Unknown) was unknown.   For mRNA-1273 (Moderna COVID-19 Vaccine) (Unknown), the reporter did not provide any causality assessments.   Expanded skin testing for COVID-19 vaccine excipients was performed. In addition, percutaneous and intradermal skin tests (1:10 dilution and full strength both for pricks and intradermals) with Moderna and Pfizer COVID-19 vaccines were completed.  We recorded whether premedication was used and analyzed available laboratory data (serum tryptase, soluble terminal complement complex, complete blood cell count with differential). Premedication could consist of H1 blockers, H2 blockers, and leukotriene antagonists. Patient outcome or vaccine tolerance was assessed through follow-up and chart review.  Findings suggest a simpler paradigm in that most individuals with previous reactions would be able to receive their subsequent COVID-19 vaccine dose with or without premedication or by treating through mild to moderate allergic symptoms.  A limitation of this study was that nonirritant skin test concentrations of COVID-19 vaccine have not yet been established. We did skin test control in individuals on our health care team, and skin test results were negative, suggesting positive immediate intradermal skin test results at 1:10 and full strength were likely nonirritant.  Our findings did not reveal an immunoglobulin E-mediated pathogenesis for immediate-type COVID-19 vaccine hypersensitivity, given the vastly negative skin test results. There may be nonspecific mast cell histamine release which can be countered with premedication or treating through the reactions.  Company comment: This is a Literature-Non-Study case concerning a patient of unknown gender and age with no medical history reported, who experienced the unexpected serious (due to medically significant) event of Systemic mastocytosis (reported as Indolent Systemic mastocytosis). The event occurred on unknown date, after the most recent dose of Spikevax (Moderna COVID-19 vaccine). The event presented as chronic urticaria starting 4 days after Moderna vaccine administration, in addition to an elevated serum tryptase level obtained weeks after this vaccine (25.7 mg/L). Subsequent bone marrow and diagnostic criteria confirmed indolent systemic mastocytosis. No information was reported about treatment, if any, and outcome. The benefit-risk relationship of Spikevax (Moderna COVID-19 vaccine) is not affected by this report.   This case was linked to US-MODERNATX, INC.-MOD-2021-341168 (Linked Report).   Most recent FOLLOW-UP information incorporated above includes: On 04-Oct-2021: Upon internal review on 02-Aug-2023, a significant correction was performed to upgrade the event Indolent systemic mastocytosis. On 06-Oct-2021: Follow up received by safety on 07-Oct-2021 has Email with Agency received from team and does not contain any new information. On 11-Oct-2021: Follow up received by safety on 13-Oct-2021 has Email with Agency received from team and the event has been updated. On 13-Oct-2021: Follow up received by safety on 14-Oct-2021 has Email with Agency received from team and does not contain any new information; Sender's Comments:  US-MODERNATX, INC.-MOD-2021-341168:Patient 1 (Different patient same article)