My friend does have Mast Cell Activation Syndrome and her allergist believes the flu vaccine caused her mast cells to overreact/flare with patient's MCAS; My friend does have Mast Cell Activation Syndrome and her allergist believes the flu vaccine caused her mast cells to overreact/flare with patient's MCAS; Broke out in hives; Itchiness; This spontaneous case was received on 08-Nov-2023 from other non-healthcare professional via Company (reference number SEQW23-04110) with additional information received on the same date (reference number: SEQW23-04111 (being processed together), and concerned a 34-year-old, female patient.  The patient's concurrent conditions included Ehler Danlos syndrome and mast cell activation syndrome. The patient's concomitant medications were not reported. The patient's historical vaccinations included Flucelvax vaccines in previous years for unspecified indication, after which the patient had no issues.  On 05-Nov-2023, reported as on Sunday, the patient was vaccinated with Flucelvax Quadrivalent (influenza vaccine inact sag 4v; dose: 0.5 ml, route of administration: intramuscular, anatomical location: not reported) as active immunization against influenza disease. The batch number reported was 944479 (reported batch number was identified by Seqirus to be correct batch number) and expiry date: 30-Jun-2024.  On an unspecified date in Nov-2023, after receiving Flucelvax Quadrivalent vaccine, the patient broke out in hives and was required to be placed on steroids. The patient had mast cell activation syndrome (MCAS) and her allergist believed that the flu vaccine caused her mast cells to overreact. Following the onset of symptoms, the patient met with allergist and was placed on steroids until the itchiness resolves. Allergist suspected flu vaccine might have caused a flare with the patient's MCAS because flare was so close to receipt of the Flucelvax vaccine.  The outcome of the events was not reported.  The reporter assessed all events as related to Flucelvax Quadrivalent vaccine. As reported, the patient had a reaction to Flucelvax Vaccine. The patient's allergist believed that, the flu vaccine caused her mast cells to overreact. Allergist suspected flu vaccine might have caused a flare with patient's MCAS because flare was so close to receipt of the Flucelvax vaccine.   The events of 'mast cell activation syndrome' and 'condition aggravated' were considered to be medically significant by a Physician within CSL Pharmacovigilance Department.  Company comment: On an unspecified date in Nov-2023, within three days after receiving Flucelvax Quadrivalent vaccine, the patient broke out in hives and was placed on steroids, until the itchiness resolved. The patient's allergist suspected flu vaccine might have caused a flare with the patient's MCAS because flare was so close to receipt of the Flucelvax vaccine, as reported. Causal role of the suspect vaccine is assessed as possible for mast cell activation syndrome/condition aggravated, urticaria and pruritus, based on plausible temporal relationship.; Reporter's Comments: As reported, the patient had a reaction to Flucelvax Vaccine. The patient's allergist believed that the flu vaccine caused her mast cells to overreact. Allergist suspected flu vaccine might have caused a flare with patient's MCAS because flare was so close to receipt of the Flucelvax vaccine.; Sender's Comments: On an unspecified date in Nov-2023, within three days after receiving Flucelvax Quadrivalent vaccine, the patient broke out in hives and was placed on steroids, until the itchiness resolved. The patient's allergist suspected flu vaccine might have caused a flare with the patient's MCAS because flare was so close to receipt of the Flucelvax vaccine, as reported. Causal role of the suspect vaccine is assessed as possible for mast cell activation syndrome/condition aggravated, urticaria and pruritus, based on plausible temporal relationship.