This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Communicable Diseases, Subject name: Social & Preventive Medicine
A 2-year-old child diagnosed provisionally with diphtheria and on examination she has greyish white membrane patch around her tonsils. The child has a 6-year-old sibling at home, who is fully immunized as per the schedule. What is the best measure to prevent disease in the sibling of the child 
Select and explain. 
A. Full course of DPT, B. Booster dose of DPT, C. Nothing is required to be done, D. Prophylactic erythromycin to be given
C: Nothing is required to be done
Answer: C: Nothing is required to be done 
Explanation: * For the contacts if the primary course of vaccination is less than 2 years- nothing is required* Vaccine is taken >2 years- 1 dose toxoid is required* Unimmunized contacts-antitoxin, active immunization, prophylactic penicillin or erythromycin and clinical observation for 1 week