Vaccination

Vaccination is the process of administering a vaccine to help the immune system develop immunity. This process helps prevent sickness from an infectious disease by stimulating adaptive immunity. Vaccines work by introducing an immunogen, which is a substance that triggers an immune response. These immunogens can consist of a microorganism or virus in a weakened, live, or killed state. They may also contain specific proteins or toxins from the organism. By using these components, the body learns to recognize and fight the actual pathogen later.

The mechanism of vaccination involves priming the immune system. This artificial activation is known as immunization. Most vaccines are administered before a person contracts a disease to build future protection. However, some are given after exposure to reduce the severity of the illness. For example, vaccines given after smallpox exposure can offer some protection. Louis Pasteur famously provided the first rabies immunization to a child bitten by a rabid dog. This rabies vaccine is effective when administered several times over 14 days alongside wound care and immune globulin. While vaccination provides lasting protection, it usually takes several weeks to develop. This is different from passive immunity, such as breastfeeding, which has an immediate effect.

There are different ways to deliver these immunogens to the body. Most vaccines are given by injection because they are not absorbed reliably through the intestines. However, some vaccines, such as those for rotavirus, polio, and certain types of typhoid or cholera, are given orally. These oral vaccines work to produce immunity directly in the bowel. It is also important to distinguish vaccination from inoculation. Inoculation, or variolation, involves using unattenuated or unweakened pathogens. In the case of smallpox, variolation used material from a patient's pustule or scab. This method was often less effective and could cause the patient to become severely ill.

The history of immunization began long before modern medicine. The first recorded use of variolation occurred in China during the 16th century to prevent smallpox. Smallpox was a deadly disease that killed 20% to 60% of infected adults. It killed over 80% of infected children. In 1796, English physician Edward Jenner invented the smallpox vaccine. He was the first to publish evidence of its effectiveness and provide production advice. Later, Louis Pasteur furthered these concepts through his work in microbiology. The term "vaccination" itself comes from the virus used to affect cows.

Vaccination is one of the most successful public health achievements in history. The World Health Organization (WHO) states that vaccination prevents 3.5 to 5 million deaths every year. A study in The Lancet estimated that vaccination prevented 154 million deaths between 1974 and 2024. This total includes 146 million deaths among children under age five. Widespread immunity helped eradicate smallpox in 1979, a disease that killed 300 to 500 million people in the 20th century. Vaccination also helped eliminate polio and tetanus from much of the world. In the United States, the CDC named vaccination one of the ten great public health achievements of the 20th century.

To ensure safety, vaccines undergo a rigorous multi-stage testing process. Before human testing, researchers test vaccines on cell cultures and animal models like mice, rabbits, and monkeys. Once approved for human trials, they enter three distinct phases. Phase I tests safety in about 20 people. Phase II expands to 50 or several hundred people to evaluate dose and effectiveness. Phase III involves hundreds to thousands of volunteers to compare vaccinated groups to unvaccinated groups. Even after approval, Phase IV trials monitor safety in tens of thousands of people over many years. While minor side effects like fever are common, serious allergic reactions occur in about 1 out of 100,000 vaccinations.

Vaccines contain various ingredients to help them work effectively. Some use an adjuvant, such as aluminium salts, to create a stronger immune response. Aluminium hydroxide or aluminium phosphate have been used safely since the 1930s. Some vaccines also contained thimerosal, an organic compound containing ethylmercury. It is important to note that ethylmercury is different from the methylmercury found in contaminated fish. Although vaccination is highly effective, "vaccine failure" can occur. This happens when an organism contracts a disease despite being vaccinated. This is often due to individual variations in immune response rather than a defective vaccine.