A 4-year-old girl from Delhi presents to the emergency department with acute respiratory distress, stridor, and inability to swallow saliva. Her mother reports a 4-day prodrome of low-grade fever and sore throat. On examination, a thick greyish pseudomembrane is visible extending from the pharynx into the larynx. A throat culture is pending. Immediate management includes antitoxin administration. Which of the following statements about diphtheria antitoxin is INCORRECT?