<prompt> Serial interval is -Interval between primary and secondary caseInterval between first case and last caseInterval between infection to maximum infectivisInterval between incubation period and gestational period  <response>: Ans. is a i.e., Interval between primary and secondary case Serial intervalo The gap time between onset of primary case and secondary case.o 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. 