This case was reported in a literature article and described the occurrence of inflammatory arthritis in a 10-day-old female patient who received Hepatitis B vaccine. The patient's past medical history included pre-eclampsia (Family History), attention deficit hyperactivity disorder (Family History), bipolar depression (Family History) (patient's mother experienced during pregnancy and treated with sertraline) and premature baby (born at 36 weeks for maternal preeclampsia).   On an unknown date, the patient received Hepatitis B vaccine. On an unknown date, 10 days after receiving Hepatitis B vaccine, the patient experienced inflammatory arthritis (serious criteria hospitalization), knee swelling (serious criteria hospitalization), knee effusion (serious criteria hospitalization), joint range of motion decreased, mobility decreased and joint stiffness. The patient was treated with ibuprofen. On an unknown date, the outcome of the inflammatory arthritis, knee effusion and joint stiffness were unknown and the outcome of the knee swelling, joint range of motion decreased, and mobility decreased were recovered/resolved. The reporter considered the inflammatory arthritis, knee swelling, knee effusion, joint range of motion decreased, mobility decreased and joint stiffness to be related to Hepatitis B vaccine. Additional details were provided as follows: This case was reported in a literature article and described the occurrence of Inflammatory Arthritis, in a 10-day-old female patient, who was vaccinated with Unspecified Hepatitis B vaccine (manufacturer unknown).   The patient was a previously healthy female presented to her pediatrician with a decreased movement of the right lower extremity. The patient was born at 36 weeks for maternal preeclampsia. The mother reported that she had regular prenatal care. The only perinatal complication was preeclampsia for which she was induced at 36-week gestation. Maternal history was significant for ADHD and bipolar depression, for which she was prescribed sertraline during pregnancy. Mother had negative tests for parvovirus, Lyme, SSA/RO, and SS-B/LA antibodies and ANA. There was no known family history of autoimmune diseases such as psoriasis, rheumatoid arthritis, or inflammatory bowel disease (IBD). No information on patient medical history or concurrent condition or concomitant medication was reported.  On an unspecified date, the patient received unspecified Hepatitis B vaccine in the right thigh at birth (administration route unknown; batch number not provided), prior to newborn nursery discharge.  On an unknown date, an unknown period after vaccination, the patient kept her right knee and hip in a flexed position since discharge. The patient primary care provider initially believed refusal to move the right lower extremity was due to right hip flexor tightness. A hip ultrasound was normal. On an unknown date, at 10-days-of life, the patient presented to the Hospital and was noted to have right knee swelling. Her initial set of labs were significant for white cell count 11,000 cells/mm3, hemoglobin 14.1 g/dl, platelets 463,000, ESR 18 mm/hr (normal less than 15 mm/hr), and C-reactive protein 4.1 mg/L (normal less than 8 mg/L). A knee magnetic resonance imaging (MRI) revealed a small moderate right knee effusion and no myositis or soft tissue fluid changes. Arthrocentesis was performed; culture was negative after 5 days. The patient remained afebrile throughout the hospital course and never received antibiotics. The patient was treated with ibuprofen 10 mg/kg/dose three times a day. Additional testing included negative Lyme titer, negative parvovirus IgG and IgM, normal ferritin, normal total IgA, IgG, and IgM levels, and negative ANA; however, the patient was found to have a positive HLA B27. The patient was discharged after 2 days following the negative evaluation and some increased knee range of motion. On follow-up, the patient was noted to have improvement of her right knee swelling and increased mobility of the right lower extremity. The patient was prescribed physical therapy. After 3 weeks of ibuprofen and physical therapy, the patient had significantly improved knee swelling and movement of the lower extremity; however, there was residual mild inflammation and stiffness of the knee. Since the patient had clinically improved, the family stopped ibuprofen despite recommendation to continue therapy. The family moved out of state shortly after the 2-month visit. Discussion with the patient's mother revealed a normal interval history, meeting all milestones and with an age-appropriate gait at age 13 months. A pediatric rheumatologist at the local hospital evaluated the patient, and no joint findings were identified. They recommended monitoring for recurrence of arthritis as the patient was positive for HLA B27. The patient mother also reported that the infant received all remaining vaccines, including the hepatitis B vaccine series, without any negative consequences. This case was considered as serious due to hospitalization. The authors commented: This case presents a very young infant who developed an inflammatory arthritis believed to be secondary to the initial hepatitis B vaccine. We present the first case in the literature of an infant who developed inflammatory arthritis soon after hepatitis B vaccination, similar to reports in the literature of adults with postvaccine articular complications. We considered the possibility that this child had a very early presentation of JIA in the presence of HLA B27 positivity. However, JIA typically would not demonstrate such significant improvement within 3 weeks using just NSAIDs. Additionally, there was no known family history of psoriasis or IBD, making other forms of JIA typically associated with HLA B27 less likely. Autoinflammatory diseases also need to be considered; however, our patient had no rash or fevers, and no such features were reported at the 13-month follow-up discussion. Molecular mimicry from adjuvants used to stimulate the immune response during vaccination may also play a role in developing autoimmunity as suggested in patients with multiple sclerosis or lupus, particularly in those who carry certain HLA haplotypes. We hypothesize there may be similar processes leading to the development of postvaccination inflammatory arthritis. It is possible that our patient had very early-onset JIA with an oligoarticular presentation. The different JIA subtypes include oligoarticular arthritis, psoriatic arthritis, and enthesitis-related arthritis (ERA). Our patient's presentation would be most consistent with oligoarticular JIA, which is most frequently seen in young females, although typically the ANA is positive rather than HLA B27. Oligoarticular JIA most frequently presents between 2 and 5 years of age. A review of the literature was completed to find the youngest documented cases of JIA. Identified studies report the youngest patients are 6 months of age typically. Some young patients are positive for HLA B27 although usually associated with the ERA subtype and not typically oligoarticular JIA as would be the diagnostic consideration in our patient. There are no reports of infants as young as our patient with JIA in the literature". The authors commented "This is the first reported case of an infant developing an inflammatory arthritis after receiving the hepatitis B vaccine. The patient most likely had vaccine-related inflammatory arthritis rather than JIA given the significant improvement after a short course of NSAIDs and lack of recurrence which would be expected if the arthropathy was due to early-onset JIA. There are reports of increased vulnerability to developing such an inflammatory or reactive arthritis in adult patients who are HLA B27 positive as in our patient. In addition, the literature suggests that the hepatitis B vaccination can unmask autoimmune processes in adult patients although this has not been reported in children. Healthcare providers must be aware of the rare occurrence of post-hepatitis B vaccine inflammatory "reactive" arthritis when caring for young children, especially those known to carry arthritis risk HLA alleles".