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
Threshold level of herd immunity of Peussis is - 
Select and explain. 
A. 80%, B. 70%, C. 90%, D. 50%
C: 90%
Answer: C: 90% 
Explanation: Park&;s texbook of Preventive and Social Medicine 23rd edition Page no: 165 The control of peussis by immunization is still an unsolved problem Even if the level of immunization reaches 100 per cent, it is possible that the disease would not be entirely eliminated because whooping cough vaccines have never been claimed to be more than 90 per cent effective.