One (0.27%) patient treated with etanercept and leflunomide being in remission on the third day after vaccination developed acute gastroenteritis, received antibiotics, and 17 days later developed macrophage activation syndrome (fever up to 39.5 °C, thrombocytopenia, hyperferritinemia, increased transaminases, lactate dehydrogenase, and C-reactive protein), which was controlled by intravenous methylprednisolone at a dose of 10 mg/kg per day, with a gradual decrease until complete withdrawal, in combination with antibiotic therapy. The gene discussed is CRP; the disease is Thrombocytopenia.