Vaccine hesitancy is a delay in acceptance, or refusal of vaccines despite availability 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. Although adverse effects associated with vaccines are occasionally observed, 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.

Vaccine hesitancy is complex and context-specific, varying across time, place and vaccines. It can be influenced by factors such as lack of proper scientifically based knowledge and understanding about how vaccines are made or work, as well as psychological factors including fear of needles and distrust of public authorities, a person's lack of confidence (mistrust of the vaccine and/or healthcare provider), complacency (the person does not see a need for the vaccine or does not see the value of the vaccine), and convenience (access to vaccines). It has existed since the invention of vaccination and pre-dates the coining of the terms "vaccine" and "vaccination" by nearly eighty years.

"Anti-vaccinationism" refers to total opposition to vaccination. Anti-vaccinationists have been known as "anti-vaxxers" or "anti-vax". The specific hypotheses raised by anti-vaccination advocates have been found to change over time. Anti-vaccine activism has been increasingly connected to political and economic goals. Although myths, conspiracy theories, misinformation and disinformation spread by the anti-vaccination movement and fringe doctors leads to 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.

Proposed laws that mandate vaccination, such as California Senate Bill 277 and Australia's No Jab No Pay, have been opposed by anti-vaccination activists and organizations. Opposition to mandatory vaccination may be based on anti-vaccine sentiment, concern that it violates civil liberties or reduces public trust in vaccination, or suspicion of profiteering by the pharmaceutical industry.

Scientific evidence for the effectiveness of large-scale vaccination campaigns is well established. It is estimated that two to three million deaths are prevented each year worldwide by vaccination, and it is thought that an additional 1.5 million deaths could be prevented each year if all recommended vaccines were used. Vaccination campaigns helped eradicate smallpox, which once killed as many as one in seven children in Europe, and have nearly eradicated polio. As a more modest example, infections caused by Haemophilus influenzae (Hib), a major cause of bacterial meningitis and other serious diseases in children, have decreased by over 99% in the US since the introduction of a vaccine in 1988. It is estimated that full vaccination, from birth to adolescence, of all US children born in a given year would save 33,000 lives and prevent 14 million infections.

There is anti-vaccine literature that argues that reductions in infectious disease result from improved sanitation and hygiene (rather than vaccination) or that these diseases were already in decline before the introduction of specific vaccines. These claims are not supported by scientific data; the incidence of vaccine-preventable diseases tended to fluctuate over time until the introduction of specific vaccines, at which point the incidence dropped to near zero. A Centers for Disease Control and Prevention website aimed at countering common misconceptions about vaccines argued, "Are we expected to believe that better sanitation caused the incidence of each disease to drop, just at the time a vaccine for that disease was introduced?"

Another rallying cry of the anti-vaccine movement is to call for randomized clinical trials in which an experimental group of children are vaccinated while a control group are unvaccinated. Such a study would never be approved because it would require deliberately denying children standard medical care, rendering the study unethical. Studies have been done that compare vaccinated to unvaccinated people, but the studies are typically not randomized. Moreover, literature already exists that demonstrates the safety of vaccines using other experimental methods.

Other critics argue that the immunity granted by vaccines is only temporary and requires boosters, whereas those who survive the disease become permanently immune. As discussed below, the philosophies of some alternative medicine practitioners are incompatible with the idea that vaccines are effective.