Vaccine hesitancy is a delay in acceptance, or refusal, of vaccines despite the availability of vaccine services and supporting evidence. The term covers refusals to vaccinate, delaying vaccines, accepting vaccines but remaining uncertain about their use, or using certain vaccines but not others. The scientific consensus that vaccines are generally safe and effective is overwhelming. Vaccine hesitancy often results in disease outbreaks and deaths from vaccine-preventable diseases. Therefore, the World Health Organization characterizes vaccine hesitancy as one of the top ten global health threats.

Anti-vaccine activism, also called the "anti-vax" movement, is organized activity to show opposition to vaccination, often designed to increase vaccine hesitancy by disseminating misinformation or disinformation. Although myths, conspiracy theories, misinformation and disinformation spread by the anti-vaccination movement and fringe doctors increase vaccine hesitancy and public debates around the medical, ethical, and legal issues related to vaccines, there is no serious hesitancy or debate within mainstream medical and scientific circles about the benefits of vaccination.