A vaccine controversy is a dispute over the morality, ethics,

effectiveness, or safety of vaccinations. The medical and scientific evidence demonstrates that the benefits of preventing suffering and death from infectious diseases outweigh rare adverse effects of immunization. Since vaccination began in the late 18th century, opponents have claimed that vaccines do not work, that they are or may be dangerous, that individuals should rely on personal hygiene instead, or that mandatory vaccinations violate individual rights or religious principles. These arguments have succeeded in reducing vaccination rates in certain communities, leading to increased outbreaks of preventable, and sometimes fatal, childhood illnesses.

Vaccines may cause side effects, and the success of immunization programs depends on public confidence in their safety. Concerns about immunization safety often follow a pattern: some investigators suggest that a medical condition is an adverse effect of vaccination; a premature announcement is made of the alleged adverse effect; the initial study is not reproduced by other groups; and finally, it takes several years to regain public confidence in the vaccine.