This case was reported in a literature article and described the occurrence of Sweet's syndrome in a 53-year-old female patient who received Influenza vaccine Quadrivalent. The patient's past medical history included infectious mononucleosis. On an unknown date, the patient received Influenza vaccine Quadrivalent at an unknown dose. On an unknown date, less than a day after receiving Influenza vaccine Quadrivalent, the patient experienced Sweet's syndrome (serious criteria GSK medically significant), necrobiosis, erythema annulare, joint range of motion decreased, swollen joints, skin plaque, rash, fatigue, fever, night sweat and arthralgia. The patient was treated with NAPROSYN, Topical Corticosteroid (Nos) and Non-Steroidal Anti-Inflammatory Drugs. On an unknown date, the outcome of the Sweet's syndrome, necrobiosis, erythema annulare, joint range of motion decreased and skin plaque were unknown and the outcome of the swollen joints, rash, fatigue, fever, night sweat and arthralgia were recovering/resolving. The reporter considered the Sweet's syndrome, necrobiosis, erythema annulare, joint range of motion decreased, swollen joints, skin plaque, rash, fatigue, fever, night sweat and arthralgia to be related to Influenza vaccine Quadrivalent. Additional information was provided. This case was reported in a literature article and described the occurrence of histiocytoid Sweet's syndrome in a 53-year-old female patient who was vaccinated with unspecified influenza vaccine (manufacturer unknown). The patient was previously healthy. The patient took no other medications, denied recent travel or exposures, had no history of cold sores or illnesses except for a remote history of mononucleosis. The patient worked in healthcare as an outpatient paediatric nurse and had received yearly flu vaccines for 20 years. The patient was up to date with malignancy screenings and her screenings were unremarkable. No information on patient's family history or concurrent condition or concomitant medication was provided. On an unspecified date, the patient received unspecified influenza vaccine (administration route and site unspecified; dosages unknown; batch number not provided). On an unspecified date, later that day after vaccination, the patient developed fatigue, low-grade fevers, night sweats, rash, arthralgia and joint swelling which improved with NAPROSYN. The patient's examination was significant for annular erythematous, edematous plaques with central clearing scattered throughout her lower extremities with bilateral ankle swelling and decreased range of motion. Skin biopsy revealed granulomatous inflammation in dermis with necrobiosis consistent with histiocytoid Sweet's syndrome. The patient's laboratory findings were significant for an elevated erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) with negative herpes simplex virus (HSV) serologies and Epstein-Barr virus (EBV) serologies consistent with a previous infection (values and normal range not provided). The patient's rash improved with topical corticosteroids and her joint swelling improved with Non-Steroidal Anti-Inflammatory Drugs (NSAIDs) and compression. The patient did not require systemic corticosteroids. The authors considered the event of histiocytoid Sweet's syndrome related to unspecified influenza vaccination. The authors concluded that "While there are several cases of vaccination-triggered Sweet's syndrome, including influenza vaccine, this case highlights Sweet's syndrome as a rare complication of vaccination. There is no clear consensus regarding the risk of recurrence with subsequent influenza vaccinations."