Inflammatory Knee Reaction Mimicking Septic Arthritis Following Closely Spaced Pneumococcal Vaccines in a Pediatric Patient Undergoing Immunodeficiency Evaluation.

Vaccines in the pediatric population are crucial in preventing infectious diseases, but rare inflammatory responses can arise, particularly in patients with suspected immunodeficiency. This case highlights a unique inflammatory reaction mimicking septic arthritis following closely spaced pneumococcal vaccines (PPSV23) in a child undergoing immunodeficiency work-up. A previously healthy five-year-old Caucasian female with a history of recurrent bacterial infections presented with a two-day history of left knee swelling and pain after receiving two pneumococcal vaccines within a five-week interval due to a scheduling error. She received the vaccine as part of a "pneumococcal challenge" to assess for an appropriate immunological response. On initial presentation, she was febrile and unable to bear weight; with laboratory findings significant for a white blood cell (WBC) count of 13.6 x10/L, erythrocyte sedimentation rate (ESR) of 26 mm/hour, and C-reactive protein (CRP) of 7.4 mg/dL. Knee joint aspiration revealed a WBC count of 13,000 x 10/L with neutrophil predominance. MRI of the knee demonstrated myositis, cellulitis, and non-necrotizing fasciitis but no evidence of septic arthritis or osteomyelitis. She received empiric intravenous antibiotics, followed by oral therapy with a plan to continue antibiotics for seven days based on her clinical response. By the time of discharge, her symptoms had fully resolved. This unique case underscores the importance of careful vaccine scheduling in the setting of immunodeficiency work-up, highlighting the potential for vaccine-induced inflammatory reactions in this patient population. Clinicians should consider post-vaccination immune responses in the differential diagnosis of acute joint swelling, especially when infectious causes are not clearly identified.