This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Epidemiology, Subject name: Social & Preventive Medicine
Serial interval is 
Select and explain. 
A. Interval between primary and secondary case, B. Interval between first case and last case, C. Interval between primary case and all subsequent cases multiplied by secondary attack rate, D. Interval between incubation period and gestational period
A: Interval between primary and secondary case
Answer: A: Interval between primary and secondary case 
Explanation: The gap in the time interval between the onset of primary case and the secondary case is known as serial interval. By collecting information about a whole series of such onset we get a distribution of secondary cases from which we can guess the incubation period of disease ( refer pg 100, park 23 rd edition )