blood pressure was also affected; difficulty with heart rate; cant think clearly; fatigue; head discomfort; nausea; diarrhea; get dizzy that night; headache; Initial information received on 11-Oct-2021 regarding an unsolicited valid serious case from a physician via non-healthcare professional/consumer via Regulatory authority (Reference number- 00807496).  This case involves a 66 year old male patient who experienced get dizzy that night (dizziness), diarrhea (diarrhoea), difficulty with heart rate (heart rate abnormal), blood pressure was also affected (blood pressure abnormal), nausea, head discomfort, headache, fatigue and cant think clearly (mental impairment), after receiving vaccine INFLUENZA QUADRIVAL A-B HIGH DOSE HV VACCINE [FLUZONE HIGH-DOSE QUADRIVALENT].  The patient's medical treatment included three weeks ago, the patient's doctors believed that the patient overdosed on Caffeine. However, he had fully recovered and that's why he was approved to get FLUZONE HD.  The patient's past vaccination included COVID (coronavirus disease)-19 VACCINE in APR-2021.  The patient's medical history, concomitant medication and family history were not provided.  On 05-Oct-2021, the patient received a 0.7 ml dose of suspect INFLUENZA QUADRIVAL A-B HIGH DOSE HV VACCINE (lot number and expiry date: not reported) (Frequency: once) via an intramuscular route in the left arm for prophylactic vaccination.  On 05-Oct-2021 (night), the patient developed a serious get dizzy (dizziness), on same day following the administration of INFLUENZA QUADRIVAL A-B HIGH DOSE HV VACCINE. The next day, the dizziness became worse. The patient was hospitalized for this event.   On 06-Oct-2021, the patient developed a serious diarrhea (diarrhoea), difficulty with heart rate (heart rate abnormal) and blood pressure was also affected (blood pressure abnormal), one day following the administration of INFLUENZA QUADRIVAL A-B HIGH DOSE HV VACCINE. The patient was hospitalized for these events.  On 07-Oct-2021 (Thursday), the patient went to the hospital. He was given electrolytes and was released.  On 09-Oct-2021 (Saturday), the patient developed a serious nausea and head discomfort, four days following the administration of INFLUENZA QUADRIVAL A-B HIGH DOSE HV VACCINE. The patient was hospitalized for these events.  In Oct-2021, the patient developed a serious headache and fatigue, few days following the administration of INFLUENZA QUADRIVAL A-B HIGH DOSE HV VACCINE. The patient was hospitalized for these events.  In Oct-2021, the patient developed a serious cant think clearly (mental impairment), few days following the administration of INFLUENZA QUADRIVAL A-B HIGH DOSE HV VACCINE. This event was assessed as medically significant. The patient was hospitalized for this event.   He was discharged today. The patient has followed up with multiple doctors in his primary care office.  Details of laboratory data not reported.  It was not reported if the patient received a corrective treatment.  At time of reporting, the outcome was unknown for the reported events.  Information on the batch number was requested.; Sender's Comments: This case involves a 66 year old male patient who had dizziness, diarrhoea, heart rate abnormal, blood pressure abnormal, nausea, head discomfort, headache, fatigue and mental impairment, after receiving FLUZONE HIGH-DOSE QUADRIVALENT. The time to onset is compatible. Medical treatment included the patient overdosed on Caffeine. The patient's past vaccination included COVID-19 VACCINE. Additional information regarding patient's medical history, condition at the time of the events, lab data excluding other predisposing etiologies would be needed for complete assessment of the case. Based upon the reported information, the role of a vaccine cannot be assessed.