Vaccination

Vaccination introduces a vaccine into the body to train the immune system to recognize and fight a specific disease. These vaccines can contain a live but weakened version of a germ, a killed version, or just pieces like proteins or toxins from it. By triggering the body’s adaptive immunity, vaccination prevents illness from infectious diseases. When enough people in a community are vaccinated, herd immunity develops, which shields those with weakened immune systems who cannot safely receive vaccines themselves.

Extensive research has confirmed that vaccination is the most powerful tool for stopping infectious diseases. This widespread immunity has wiped out smallpox globally and eliminated diseases like polio and tetanus from many regions. The World Health Organization estimates that vaccination prevents 3.5 to 5 million deaths each year. A study funded by the WHO and published in *The Lancet* calculated that over the 50 years starting in 1974, vaccination prevented 154 million deaths, 146 million of which were children under five. However, some diseases are making a comeback in areas where vaccination rates have dropped, often due to vaccine hesitancy.

The earliest known attempt to prevent a disease through inoculation was for smallpox, with records from 16th-century China describing variolation. Smallpox was also the first disease for which a true vaccine was created. Though at least six people before him had used similar methods, English physician Edward Jenner is credited with inventing the smallpox vaccine in 1796. He was the first to publish proof that it worked and to give instructions for making it. Louis Pasteur later expanded on these ideas through his microbiology research. The term “vaccination” comes from the Latin word for cow, because the vaccine was derived from a cowpox virus. Smallpox was a devastating illness, killing 20 to 60 percent of infected adults and over 80 percent of infected children. By the time it was declared eradicated in 1979, it had killed an estimated 300 to 500 million people in the 20th century alone.

In everyday language, “vaccination” and “immunization” are used similarly, but they differ from “inoculation,” which involves using live, unweakened pathogens. Vaccination has faced resistance for scientific, ethical, political, medical, and religious reasons, though no major religion forbids it, and some consider it a moral duty because it saves lives. In the United States, a compensation program exists for people who claim injury from a vaccine. Early success led to widespread acceptance, and mass vaccination campaigns have sharply reduced many diseases across the globe. The US Centers for Disease Control and Prevention ranks vaccination as one of the ten greatest public health achievements of the 20th century in America.

Vaccines work by artificially priming the immune system with an immunogen, a process called immunization. Most vaccines are given before a person catches a disease, but some are used after exposure. For example, the smallpox vaccine can still offer some protection or reduce severity even after infection. Louis Pasteur gave the first rabies vaccine to a child bitten by a rabid dog; since then, a series of shots over 14 days, combined with rabies immune globulin and wound care, has proven effective. Researchers are also testing experimental vaccines for AIDS, cancer, and Alzheimer’s, aiming to trigger a faster, safer immune response than natural infection.

Most vaccines are injected because they are not absorbed well through the intestines. However, live attenuated polio, rotavirus, and some typhoid and cholera vaccines are taken orally to build immunity in the gut. Vaccination provides lasting protection, but it usually takes weeks to develop, unlike passive immunity—such as antibodies from breastfeeding—which works immediately.

A vaccine failure happens when a person gets sick despite being vaccinated. Primary failure occurs when the immune system does not produce antibodies after the first dose, and sometimes multiple doses still fail. This does not mean the vaccine is defective; most failures stem from natural differences in individual immune responses.

The terms “vaccination” and “inoculation” are often used interchangeably, but they are not the same. Vaccination uses a weakened or harmless version of a pathogen, while inoculation—called variolation for smallpox—involves using live, unweakened virus from a smallpox patient’s pustule or scab, applied to the skin, usually the upper arm. Variolation was often done directly from person to person or, less effectively, from scab to person.