This spontaneous case was reported by a consumer and describes the occurrence of CARDIAC DISORDER (My heart went haywire), DIZZINESS (I was dizzy) and GAIT INABILITY (I couldn't walk like my body just did things that the doctor said was not supposed to happen with that shot) in a 33-year-old female patient who received mRNA-1273 (Moderna COVID-19 Vaccine) (batch no. 04IL20A) for COVID-19 vaccination. The occurrence of additional non-serious events is detailed below. No Medical History information was reported. On 13-Jan-2021, the patient received dose of mRNA-1273 (Moderna COVID-19 Vaccine) (Intramuscular) 1 dosage form. On 13-Jan-2021 at 12:00 PM, the patient experienced CARDIAC DISORDER (My heart went haywire) (seriousness criterion hospitalization), DIZZINESS (I was dizzy) (seriousness criterion hospitalization) and GAIT INABILITY (I couldn't walk like my body just did things that the doctor said was not supposed to happen with that shot.) (seriousness criterion hospitalization). On an unknown date, the patient experienced INTERCHANGE OF VACCINE PRODUCTS (patient received Moderna COVID-19 vaccine & Pfizer COVID-19 vaccine; mixed dose because of my reaction to Moderna). The patient was hospitalized from 13-Jan-2021 to 18-Jan-2021 due to CARDIAC DISORDER, DIZZINESS and GAIT INABILITY. On 25-Jun-2021, CARDIAC DISORDER (My heart went haywire), DIZZINESS (I was dizzy) and GAIT INABILITY (I couldn't walk like my body just did things that the doctor said was not supposed to happen with that shot) had resolved with sequelae. At the time of the report, INTERCHANGE OF VACCINE PRODUCTS (patient received Moderna COVID-19 vaccine & Pfizer COVID-19 vaccine; mixed dose because of my reaction to Moderna) had resolved. The action taken with mRNA-1273 (Moderna COVID-19 Vaccine) (Intramuscular) was unknown. No concomitant medications were reported. No treatment medications were reported. Patient took a mixed dose because of her reaction to Moderna. She thought it would be different with Pfizer. Company Comment: This is a spontaneous case of interchange of vaccine products for this 33-year-old, female patient with history of administration of a dose of the Pfizer/BioNTech COVID-19 mRNA vaccine, who experienced the unexpected, serious (hospitalization) events of cardiac disorder, dizziness and gait inability. The events occurred on the same day after administration of an unspecified dose of the Moderna mRNA-1273 vaccine. The patient stated that her heart went 'haywire'. She was dizzy and could not walk, it was 'like my body just did things that the doctor said was not supposed to happen' following vaccination. On the same day, the patient sought consultation at the Emergency department and was subsequently admitted for 6 days. No further clinical information (including details about hospitalization) was available for medical review. Treatment information was also not provided. However, the events were reported to have resolved with sequelae at the time of the report. The benefit-risk relationship of the Moderna mRNA-1273 vaccine is not affected by this report. Further information has been requested. Most recent FOLLOW-UP information incorporated above includes: On 19-Apr-2022: Follow-up included case upgraded to serious, patient details, events, event start date, stop date, outcome and suspect details added. Sender's Comments: This is a spontaneous case of interchange of vaccine products for this 33-year-old, female patient with history of administration of a dose of the Pfizer/BioNTech COVID-19 mRNA vaccine, who experienced the unexpected, serious (hospitalization) events of cardiac disorder, dizziness and gait inability. The events occurred on the same day after administration of an unspecified dose of the Moderna mRNA-1273 vaccine. The patient stated that her heart went 'haywire'. She was dizzy and could not walk, it was 'like my body just did things that the doctor said was not supposed to happen' following vaccination. On the same day, the patient sought consultation at the Emergency department and was subsequently admitted for 6 days. No further clinical information (including details about hospitalization) was available for medical review. Treatment information was also not provided. However, the events were reported to have resolved with sequelae at the time of the report. The benefit-risk relationship of the Moderna mRNA-1273 vaccine is not affected by this report. Further information has been requested.