BACKGROUND: COVID-19 mRNA vaccination-associated acute-onset hypersensitivity reactions have caused anxiety and may be contributing to vaccine hesitancy. OBJECTIVE: To determine the incidence, severity, and risk factors for treated acute-onset COVID-19 mRNA vaccination-associated hypersensitivity reactions in a well-characterized population. METHODS: All Kaiser Permanente Southern California (KPSC) members who received COVID-19 mRNA vaccinations between December 15, 2020, and March 11, 2021, at a KPSC facility were identified and characterized, along with all treated acute-onset vaccination-associated hypersensitivity events. RESULTS: We identified 391,123 unique vaccine recipients (59.18% female, age 64.19 ± 17.86 years); 215,156 received 2 doses (53.54% Moderna), 157,615 only a first dose (50.13% Moderna) (1961 [1.46%] >2 weeks late getting a second dose), and 18,352 (74.43% Moderna) only a second dose. Only 104 (0.028%) (85.58% female, age 53.18 ± 15.96 years) had treated first dose events, 68 (0.030%) Moderna. Only 32 (0.014%) (93.75% female, age 57.28 ± 17.09 years) had treated second dose events, 21 (0.016%) Moderna. Only 2 (0.00033%) vaccinations resulted in anaphylaxis. Only 27 (20.77%) of those with treated first dose reactions failed to get a second dose. Only 6 of 77 (7.8%) with first dose reactions also had second dose reactions. Individuals with treated events were more likely to be female (P < .0001), younger (P < .0001), and had more pre-existing drug "allergies" (2.11 ± 2.12 vs 1.02 ± 1.41 [P < .0001] for average recipients). CONCLUSIONS: Treated acute-onset hypersensitivity events were mostly benign, more common with first COVID-19 mRNA vaccine doses, more likely to occur in younger females with typical risk factors associated with multiple drug intolerance syndrome, and very unlikely to be primarily immunologically mediated.