Non ischemic heart failure; This spontaneous case was received on 21-Jul-2021 from other health professional (nurse) via Med Communications (reference number: SEQW21-01509) and concerned a 55-year-old, female patient.  The patient's concurrent conditions included hypothyroidism. The patient's concomitant medications included levothyroxine used for an unreported indication.  On 07-Oct-2019, the patient was vaccinated with Flucelvax Quadrivalent (influenza vaccine, subunit influenza virus vaccine polyvalent; dose: 0.5 ml, route of administration: intramuscular, anatomical location: deltoid and indication: not reported). The batch number reported was 261208 and expiry date 30-Jun-2020.  On 21-Oct-2019, two weeks after receiving Flucelvax Quadrivalent, the patient developed signs of non-ischemic heart failure. The patient had shortness of breath and was snoring loudly which she did not have prior to mid-October.   On an unspecified date in 2019, electrocardiogram (EKG) revealed heart damage and ejection fraction was at 20 %. The patient then went to emergency room and was admitted to the hospital. The doctors could not determine the cause of heart failure despite the patient had been tested for multiple things, and no infection or medical reason was found except that she was administered the flu vaccine two weeks prior. The patient was prescribed with Entresto (sacubitril valsartan sodium hydrate), carvedilol, spironolactone and Bumex (bumetanide) as corrective treatment.   On an unspecified date, the patient was discharged from the hospital.   At the time of initial reporting on 21-Jul-2021, the patient had not recovered from the event. As reported the heart failure was resolved with medications, this was interpreted as the patient was stable without symptoms while consuming medications. Since the event was not reversible by nature, the outcome was recorded as unresolved.   The reporter assessed the event as related. The patient had been tested for multiple things and could find no infection or medical reason except that she was administered the flu vaccine two weeks prior.   The reporter assessed the event of heart failure as serious due to hospitalisation. Also, this event was considered to be medically significant by a Physician within Seqirus' Pharmacovigilance and Risk Management Department.  Company comment: A 55-year-old, female patient was vaccinated with the suspect product Flucelvax Quadrivalent. Two weeks after vaccination, the patient developed signs of non-ischemic heart failure. Electrocardiogram (EKG) revealed heart damage and ejection fraction was at 20%. As reported, the doctors could not determine the cause of heart failure although multiple tastings have been performed. More information regrading the results of related diagnostic findings is needed. Due to limited information regarding diagnostic data, causality for the reported event is unassessable.; Reporter's Comments: The patient had been tested for multiple things and could find no infection or medical reason except that she was administered the flu vaccine two weeks prior.; Sender's Comments: A 55-year-old, female patient was vaccinated with the suspect product Flucelvax Quadrivalent. Two weeks after vaccination, the patient developed signs of non-ischemic heart failure. Electrocardiogram (EKG) revealed heart damage and ejection fraction was at 20%. As reported, the doctors could not determine the cause of heart failure although multiple tastings have been performed. More information regrading the results of related diagnostic findings is needed. Due to limited information regarding diagnostic data, causality for the reported event is unassessable.