Some commonly encountered adverse events, such as fever and injection site reaction, are easily observable and documented in vaccine clinical trials however, a small number of extremely rare and sometimes potentially life-threatening adverse events may not be seen in such trials in spite of their large sample size. Pre-licensure vaccine clinical trials usually involve selected populations who receive the vaccine according to a protocol-defined administration method and who are followed for a limited period after vaccination. Due to obvious limitations, findings in animal studies can only provide biological clues on potential adverse events that may not extrapolate to humans. Vaccine safety evaluation involves pre-clinical animal studies, clinical trials and post-licensure surveillance. Hence, ensuring vaccine safety is important in public health activities and policies. In addition, many vaccines are universally recommended and mandated for schooling and some special programs (e.g., military service), where the tolerance of vaccine risk is low. Second, unlike any therapeutic products, vaccines are typically given to healthy people and even to vulnerable populations such as infants and young children.
First, the withdrawal of a rotavirus vaccine (a tetravalent rhesus-human reassortant rotavirus vaccine, RRV-TV) in 1999 has raised public concerns on vaccine safety and hence the balance of benefit and risk of a vaccine product. Despite the significant public health impact seen from the introduction of vaccines, the safety of vaccines continues to receive considerable attention, and has raised a variety of issues.
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