Safety considerations for new vaccine development.

Vaccines are highly effective and extremely safe. Although most known vaccine reactions are minor (e.g. fever, injection site pain or swelling), rare but serious reactions such as vaccine-associated paralytic polio do occur. When large populations are vaccinated, some adverse health events may occur by chance shortly after vaccination. It is difficult to determine whether these are truly coincidental or attributable to the vaccine. The most reliable way to assess causality is in a controlled study, but clinical trials of new vaccines are typically too small to detect rare but serious effects. If the size of these trials were increased, much more could be learned about the safety of a vaccine prior to its exposure to entire populations. This information would increase confidence in the safety of vaccines, would be a valuable resource for assessing spontaneous reports of adverse events after licensure, and would reduce the risk of licensing a new vaccine that had the potential to cause severe injury to a small proportion of vaccinees.