Vaccination

A vaccine is a preparation designed to train the immune system to respond to a specific disease-causing organism. Vaccination describes receiving a vaccine; immunization often refers to the resulting protection, though the words are sometimes used more broadly in public health. Unlike treatment given after illness begins, vaccination aims to prepare immune defenses before exposure. Protection varies by vaccine and person, and some vaccines require more than one dose or later boosters.

The immune system detects features of a pathogen called antigens. Vaccines present an antigen, or provide genetic instructions for cells to make one, in a way intended to stimulate an immune response without the dangers of a full natural infection. The response may produce antibodies and immune cells that retain a memory of the target. If the person later encounters the pathogen, this learned response can act more quickly. Vaccines do not all provide complete protection, and the time needed to develop protection differs among them.

Vaccine technologies include weakened live organisms, inactivated organisms, selected parts of a pathogen, and inactivated toxins. Some newer platforms deliver genetic instructions, such as RNA or DNA, that prompt the body to produce a target antigen. Vaccines can be given by injection, by mouth, or as a nasal spray. The appropriate formulation and number of doses depend on the vaccine and the disease it targets.

Vaccination reduces the likelihood of illness and can lessen the risk of severe outcomes, but it is not a guarantee that infection cannot occur. When vaccination reduces how readily an infection spreads, it can also lower exposure in a community and indirectly help protect people who cannot receive a particular vaccine. This population effect is often called community or herd protection. Its strength depends on the disease, vaccine performance, and how widely protection is shared; it does not provide complete protection to every person.

Practices that preceded modern vaccination included variolation, in which material from smallpox sores was deliberately introduced to induce protection. It was used in several regions, but because it involved the smallpox agent it carried risks. In 1796, English physician Edward Jenner tested the protective effect of cowpox against smallpox, an important milestone in the development of vaccination. Centuries later, international coordination combined vaccination with surveillance and containment; the World Health Organization declared smallpox eradicated in 1980.

Vaccines are evaluated and monitored for safety, and their benefits and possible adverse effects are considered for each product and context. A vaccine can cause a temporary immune response, and protection may be incomplete or wane over time. Disease risks, vaccine formulations, and recommended schedules differ across countries and populations, so current guidance from public-health authorities and qualified health professionals is important.