A Study to Compare the Efficacy and Safety of 2 Dosing Regimens of IV Infusions of AZD9773 (CytoFab™) With Placebo in Adult Patients With Severe Sepsis and/or Septic Shock

Inclusion Criteria: * Adults with a first episode of sepsis during this hospitalisation and objective evidence of infection that requires parenteral antibiotics. * At least 2 of 4 SIRS criteria in the 24 hours before organ dysfunction (must include either fever OR elevated white blood cells \[WBC\]) * Cardiovascular or respiratory dysfunction. Exclusion Criteria: * Immunocompromising comorbidities or concomitant medications: 1. Advanced human immunodeficiency virus (HIV) infection (CD4 ≤50/mm3). 2. Stage III or IV cancer. 3. Haemopoietic or lymphoreticular malignancies not in remission. 4. Receiving radiation therapy or chemotherapy. 5. Stem cell, organ or bone marrow transplant in the past 6 months. 6. Absolute neutrophil count \<500 per μL. 7. High dose steroids or other immunocompromising drugs. * Concomitant diseases: 1. Deep seated fungal infection or active tuberculosis. 2. Cirrhosis with portal hypertension or Childs-Pugh Class C. 3. History of chronic hypercarbia, respiratory failure in past 6 months or use of home oxygen in the setting of severe chronic respiratory disease. 4. Neuromuscular disorders that impact breathing/spontaneous ventilation. 5. Quadriplegia. 6. Cardiac arrest in the past 30 days. 7. New York Heart Association functional Class IV due to heart failure or any disorder. 8. Burns over \> 30% of body surface area. * Medication and allergy disqualifications. 1. Treatment with anti-TNF agents within the last 8 weeks. 2. Previously received ovine derived products (CroFab™, DigiFab™). 3. Sheep product allergy or allergy to latex, papain, chymopapain.