Septic arthritis of the glenohumeral joint; Rotator cuff tear; Improper administration technique for an influenza vaccine; This is a literature case, initially received on 09-Aug-2019, concerning a 32-year-old female patient.   A 32-year-old, right-hand-dominant, female paediatric intensive care unit nurse presented with the acute onset of left shoulder pain in duration of two weeks after receiving an influenza vaccine on unspecified date.   She complained of moderate anterior shoulder pain, which was deep within the joint. On examination, there was no atrophy, skin changes, or crepitus present. On range of motion testing, she had 70 degrees of external rotation (ER), 160 degrees of forward elevation (FE), and reached to T7 on internal rotation (IR), which was equal to the contralateral side. However, there was pain at endpoint of range of motion that was only present on the left. The patient described subjective tightness at the end of motion. Strength was normal and symmetric. On further testing, Hawkins, Speeds, and OBriens tests were positive. Radiographs showed no osseous abnormalities. Potential diagnosis at that time included early-stage adhesive capsulitis versus rotator cuff impingement. Physical therapy was initiated at that time.   The patient returned to the office one months later with continued pain, requiring ibuprofen and acetaminophen. Repeat range of motion testing showed 70 degrees of ER, 160 degrees of FE, and L3 on IR. Strength was still normal. No constitutional symptoms were reported. Conservative management was continued.   Three months after her original presentation, the patients symptoms were not improving. Her physical examination had not changed since the previous office visit. A magnetic resonance imaging (MRI) was obtained and showed a high-grade articular-sided tear of the supraspinatus, prominent subcortical cystic changes of the posterolateral humeral head, and a large joint effusion with debris and synovitis. The decision was made to proceed with surgery as the degree of synovitis was worrisome in a healthy 32-year-old female with no other medical comorbidities. Intraoperatively, a fibrous quality of the joint fluid was noted, but there was no purulence. The thickened joint fluid was collected and sent for cell count, crystals, Gram stain, and cultures. Upon initial visualization of the joint, a synovitis reaction was seen; the synovium was inflamed and adherent to the capsule throughout the entire shoulder. The synovitis was debrided and sent for pathology, aerobic, and anaerobic cultures. A thorough incision and drainage was performed. A deep fissure was noted between the junction of the supraspinatus and infraspinatus tendon insertion on the articular side. This was repaired with a side-to-side suture of 1 proline, which was tied on the bursal side. The long head of the biceps (LHB) tendon was erythematous, thickened, and very scarred to the rotator interval. To eliminate persistent biceps pain, a subpectoral tenodesis was performed, tendering the LHB to the pectoralis major with 1 proline suture.  Surgical pathology showed polypoid fragments of synovium and fibrovascular tissues with active chronic inflammation and focal fibrinopurulent exudate. Gram stain of the fluid collected showed gram-positive cocci, but cultures were negative. A peripherally inserted central catheter (PICC) line was placed, and the patient was started on empiric therapy with vancomycin.   The patient was seen after two weeks postoperatively and reported moderate pain. The incisions were well healed with no signs of infection. Physical therapy was started at that time. At fifth week postoperatively, her pain was improving. Range of motion of the affected shoulder was 40 degrees of ER, 140 degrees of FE, and internal rotation was not tested.   At six weeks postoperatively, the patient complained of diarrhoea. She had a history of C. difficile infection five year ago. A stool assay for C. diff toxin was positive, and she was started on a 10-day course of metronidazole. Vancomycin was stopped, and the PICC was removed after completion of six of antibiotics.  Three months postoperatively, her pain was adequately controlled without the need for medication. Range of motion was 60 degrees of ER, 160 degrees of FE, and internal rotation to L3 with mild weakness in FE on the left. She returned to work as a nurse at that time, with a 10-pound lifting restriction.   The patient returned for follow-up at 7 months postoperatively. There was still some lingering pain and tightness. Range of motion was 65 degrees ER, 160 degrees FE, T12 for IR and 60 degrees of internal rotation at 90 degrees of abduction. At that time, a radiograph showed two subtle areas of lucency. An MRI was obtained to rule-out subclinical infection, which showed no ominous signs, including interval resolution of oedema and an intact rotator cuff.   She was seen in clinic at 14 months postoperatively. At that time, she felt some weakness and fatigue, but did not experience rest pain. She continued to work full-time without restrictions. On examination range of motion was 70 degrees of ER, 170 degrees of FE, and to T12 on IR. Her strength was 5/5 with ER, 5-/5 with FE, and 5/5 with IR. The patient was satisfied with her progression and was followed on an as-needed basis after this point.  It was reported that an indolent infection, rotator cuff tear and the resulting complications were caused by improper administration technique for an influenza vaccine.  Authors comment: Our patient was a 32-year-old healthy woman with an intact immune system. She had no ongoing infections or risk factors for a joint infection. There was no previous trauma or procedure to the affected shoulder. Her pain started soon after the injection and never resolved prior to presentation. Due to a lack of another cause and the time course in which her symptoms began, we believe that the vaccine caused an indolent infection. The vaccine was the inciting event that caused the infection and the subsequent complications. The rotator cuff tear seen in our 32-year-old patient was likely due to the infection and not a chronic degenerative tear.  Company comment: Patient was diagnosed with septic arthritis following reported improper administration technique of an unspecied influenza vaccine. Authors concluded that rotator cuff tear was likely result of septic arthritis. No alternative aetiology was provided. Based on the available information the events of septic arthritis and rotator cuff tear were assessed as related to the suspect product. The event of improper administration technique of vaccine was assessed as unrelated per company convention.; Reporter's Comments: Patient was diagnosed with septic arthritis following reported improper administration technique of an unspecied influenza vaccine. Authors concluded that rotator cuff tear was likely result of septic arthritis. No alternative aetiology was provided. Based on the available information the events of septic arthritis and rotator cuff tear were assessed as related to the suspect product. The event of improper administration technique of vaccine was assessed as unrelated per company convention.