A vaccine is a biological preparation that improves immunity to a particular disease. Vaccines are among the most cost-effective and prevalent public health interventions. Morbidity & mortality declined where immunizations are practiced. No vaccine is completely safe or completely effective, while all known vaccine adverse events are minor and self-limited, some vaccines have been associated with rare but serious health effects. To overcome these limitations: Epidemiology is vital in providing scientific methodology for assessing vaccine safety. Since vaccines can lead to series of outcomes, cohort studies can be considered. In this adverse events & person-time risks are evaluated. When outcomes are rare, these studies become time consuming and expensive.