get worst symptoms after vaccination with Moderna COVID-19 vaccine; Irregular heart beat 24 - 48 hours after first booster dose; Mild Myocarditis symptoms 24 - 48 hours after first booster dose; Shortness Of Breath 24 - 48 hours after first booster dose; Heart pounding / Fast Heart beat 24 - 48 hours after first booster dose; High Blood Pressure (less than 150 Systolic pressure)  24 - 48 hours after first booster dose; Stress 24 - 48 hours after first booster dose; Fatigue 24 - 48 hours after first booster dose; second shot On 01MAR2021 and First Booster dose On 16APR2021; This spontaneous case was reported by a patient and describes the occurrence of ARRHYTHMIA (Irregular heart beat 24 - 48 hours after first booster dose) and MYOCARDITIS (Mild Myocarditis symptoms 24 - 48 hours after first booster dose) in a 67-year-old female patient who received mRNA-1273 (Moderna COVID-19 Vaccine) (batch nos. 011A21A and 041L20A) for COVID-19 vaccination. The occurrence of additional non-serious events is detailed below.     Previously administered products included for Antihistamine therapy: Zyrtec (Antihistamine Zyrtec patient took it before the first 2 doses and nothing happened). Past adverse reactions to the above products included No adverse event with Zyrtec. Concurrent medical conditions included Mast cell activation syndrome since 2018, Histamine intolerance, Cardiac disorder (Cardiologist cleared her heart (normal Heart) and it may be due to MCAS) since 2018 and Hypothyroidism (for many years and takes 25MCG Levothyroxine). Concomitant products included CETIRIZINE HYDROCHLORIDE (ZYRTEC [CETIRIZINE HYDROCHLORIDE]) for Antihistamine therapy, LEVOTHYROXINE for Hypothyroidism.   On 01-Feb-2021, the patient received first dose of mRNA-1273 (Moderna COVID-19 Vaccine) (unknown route) 1 dosage form. On 01-Mar-2021, received second dose of mRNA-1273 (Moderna COVID-19 Vaccine) (unknown route) dosage was changed to 1 dosage form. On 16-Apr-2021, received third dose of mRNA-1273 (Moderna COVID-19 Vaccine) (unknown route) dosage was changed to 1 dosage form. On 16-Apr-2021,  after starting mRNA-1273 (Moderna COVID-19 Vaccine), the patient experienced INAPPROPRIATE SCHEDULE OF PRODUCT ADMINISTRATION (second shot On 01MAR2021 and First Booster dose On 16APR2021). On 17-Apr-2021, the patient experienced ARRHYTHMIA (Irregular heart beat 24 - 48 hours after first booster dose) (seriousness criterion medically significant), MYOCARDITIS (Mild Myocarditis symptoms 24 - 48 hours after first booster dose) (seriousness criterion medically significant), DYSPNOEA (Shortness Of Breath 24 - 48 hours after first booster dose), PALPITATIONS (Heart pounding / Fast Heart beat 24 - 48 hours after first booster dose), HYPERTENSION (High Blood Pressure (less than 150 Systolic pressure)  24 - 48 hours after first booster dose), STRESS (Stress 24 - 48 hours after first booster dose) and FATIGUE (Fatigue 24 - 48 hours after first booster dose). On an unknown date, the patient experienced CONDITION AGGRAVATED (get worst symptoms after vaccination with Moderna COVID-19 vaccine). On 22-Apr-2021, ARRHYTHMIA (Irregular heart beat 24 - 48 hours after first booster dose), MYOCARDITIS (Mild Myocarditis symptoms 24 - 48 hours after first booster dose), DYSPNOEA (Shortness Of Breath 24 - 48 hours after first booster dose), PALPITATIONS (Heart pounding / Fast Heart beat 24 - 48 hours after first booster dose), HYPERTENSION (High Blood Pressure (less than 150 Systolic pressure)  24 - 48 hours after first booster dose), STRESS (Stress 24 - 48 hours after first booster dose) and FATIGUE (Fatigue 24 - 48 hours after first booster dose) had resolved. At the time of the report, INAPPROPRIATE SCHEDULE OF PRODUCT ADMINISTRATION (second shot On 01MAR2021 and First Booster dose On 16APR2021) and CONDITION AGGRAVATED (get worst symptoms after vaccination with Moderna COVID-19 vaccine) outcome was unknown.      DIAGNOSTIC RESULTS (normal ranges are provided in parenthesis if available): In April 2021, Blood pressure measurement: thought it was 2nd to stress (less than 150 Systolic pressure) for 5 days. In April 2021, Heart rate: does not remember her pulse reading, but it did not go over 100BPM..     For mRNA-1273 (Moderna COVID-19 Vaccine) (Unknown), the reporter did not provide any causality assessments.   Reporter was a Veterinary doctor. Patient never had COVID Disease. Patient spoke to the cardiologist over the phone and MD stated that it sounds like a mild case of Myocarditis. No treatment information was reported.  Company Comment: This is a spontaneous case, concerning a 67-year-old, female patient with medical history of unspecified Cardiac disorders and Mast Cell Activation Syndrome, who experienced the serious, due to medically significant, unexpected AESI of Arrhythmia and the medically significant expected AESI of Myocarditis one day after the administration of the 1st booster dose of the dose of mRNA-1273 vaccine. Patient states that 24 - 48 hours after first booster dose, she started having "mild Myocarditis symptoms" for 5 days. She consulted by phone to a cardiologist who stated that those symptoms may suggest Myocarditis. The symptoms included Shortness of Breath, Fast Heartbeat, Irregular heartbeat, Fatigue, heart pounding, High Blood Pressure (less than 150 Systolic pressure) for 5 days after first booster, pulse not over 100 BPM.  Patient refers that she gets worst symptoms after vaccination with Moderna COVID-19 vaccine, but previous cardiac disorder was not specified. The reported medical history remains as a confounder for the event. No further clinical information was available for medical review. The benefit-risk relationship of mRNA-1273 vaccine is not affected by this report.; Sender's Comments: This is a spontaneous case, concerning a 67-year-old, female patient with medical history of unspecified Cardiac disorders and Mast Cell Activation Syndrome, who experienced the serious, due to medically significant, unexpected AESI of Arrhythmia and the medically significant expected AESI of Myocarditis one day after the administration of the 1st booster dose of the dose of mRNA-1273 vaccine. Patient states that 24 - 48 hours after first booster dose, she started having "mild Myocarditis symptoms" for 5 days. She consulted by phone to a cardiologist who stated that those symptoms may suggest Myocarditis. The symptoms included Shortness of Breath, Fast Heartbeat, Irregular heartbeat, Fatigue, heart pounding, High Blood Pressure (less than 150 Systolic pressure) for 5 days after first booster, pulse not over 100 BPM.  Patient refers that she gets worst symptoms after vaccination with Moderna COVID-19 vaccine, but previous cardiac disorder was not specified. The reported medical history remains as a confounder for the event. No further clinical information was available for medical review. The benefit-risk relationship of mRNA-1273 vaccine is not affected by this report.