Vaccination is the administration of a vaccine to prime the immune system, generating immunity and enabling herd immunity to protect those who cannot be vaccinated. It stands as the most effective method for preventing infectious diseases, leading to the global eradication of smallpox (declared in 1979) and the elimination of diseases like polio and tetanus from much of the world, with the WHO crediting it with preventing 3.5–5 million deaths each year. The first vaccine was created by Edward Jenner in 1796 for smallpox—a disease that killed an estimated 300–500 million people in the 20th century—though variolation was practiced in 16th-century China. A Lancet study estimates that vaccination prevented 154 million deaths from 1974 to 2024, yet vaccine hesitancy has led to rising cases of some preventable diseases. Unlike inoculation, which uses unweakened pathogens, vaccination uses weakened or killed forms, and while it has faced some reluctance, no major religions oppose it, and it is considered a moral obligation by some. Recognized by the CDC as one of the ten great public health achievements of the 20th century, the US also provides compensation for alleged injuries via the National Vaccine Injury Compensation Program.