Serial interval is the gap between-
Primary and secondary case
Index and primary case
Introduction of infection and development of maximum infectivity
Transmission infection from patient to another susceptible host
.serial interval is the time gap between the development of primary case and the fuher introduction of the disese agent into the community to cause secondary infections.from this we get a distribution of of secondary cases from which we can guess the incubation period of disease. ref:park&;s textbook,ed22,pg 95
Epidemiology