National immunization program is a simple and effective measure to improve children's health, reduce child mortality, and prevent and reduce the occurrence of infectious diseases. Although the vaccine has undergone phase I ~ III clinical trials before it is marketed, due to the limitations of sample size and observation time, many adverse reactions cannot be known before the vaccine is marketed. Therefore, it is particularly necessary to carry out active safety monitoring based on a large population after the vaccine is marketed. self-controlled case series (SCCS) design is a common active surveillance method. SCCS was proposed in the 1990s, and this method is mainly used for safety studies of vaccines. The SCCS explores the association between time-varying exposure and adverse outcomes by containing only data from cases (patients with targeted outcomes). Since only cases were included in the SCCS, the absolute incidence rate and therefore the relative hazard or hazard ratio of outcome measures for cohort studies and the odds ratio of outcome measures for case-control studies could not be obtained. By comparing the number of events occurring in the same individual during the risk period and the control period, SCCS can obtain the relative incidence (RI).
