Vaccines and Immunization

Vaccination (vak-sih-NAY-shun) is a way of producing immunity * to a specific disease by introducing into a person's body an inactive, altered, or weakened form of a microorganism * , which provokes an immune response.

Vaccination Development

Before the 19th century, a dangerous disease known as smallpox killed millions of people throughout the world. All of that was soon to change as the result of an observation made by Edward Jenner (1749–1823), an English country doctor, in 1796. Jenner reported that milkmaids who milked cows infected with a disease known as cowpox did not contract smallpox. Instead, the milkmaids had a mild case of a similar rash-producing disease. Jenner concluded that cowpox must somehow have protected these milkmaids against the smallpox infection.

Although Jenner is credited with discovering the principles of immunization and developing the first vaccine against smallpox, men and women had grasped or intuited the principles of immunization for at least several thousand years before him. There are historical accounts of successful vaccinations against contagious illnesses in ancient Greece, India, and China. In England, approximately 75 years before Jenner's pioneering work, Lady Mary Wortley Montagu (1689–1762), wife of the British ambassador to the Ottoman Empire, vigorously promoted the practice of variolation (administering material from smallpox lesions to healthy people to produce immunity to smallpox). Montagu had observed this practice while living in what later became Turkey. She studied the method of Turkish women, who for centuries had collected in walnut shells the fluid from lesions of persons with smallpox and used lancets to press this material into the skin of healthy people to protect them from the disease. Back in England, she enthusiastically promoted this method and witnessed its many successes.

As an experiment, Jenner extracted material from the blister-like lesions of persons with cowpox and placed this material into healthy people. Jenner deliberately gave people the mild cowpox infection, which was an experiment with vaccination. He discovered that the body's natural defense system can be stimulated to become protective against a specific illness after having been ill with a similar and more benign *

After Jenner's time, scientists developed many effective vaccines, including ones against poliomyelitis * , measles * (rubeola), diphtheria * , and pertussis * . Overall, these vaccinations prevented disease and saved hundreds of millions of lives.

What Are Vaccines?

Vaccines, the preparations used in vaccinations, stimulate the body's immune response by mimicking the substances that cause disease. Small amounts of antigenic material (the vaccines themselves) are administered by oral ingestion or injection (usually intramuscular injection). Vaccines mimic a natural infection without actually causing disease. The antigenic substances promote immunologic memory, whereby specialized cells of the immune system become memory cells. This means they are able to recognize and respond to any substance that bears the same antigen * (if and when the actual disease-causing agent enters the body). When a person's immune system * can respond rapidly and effectively to an infection that is just beginning, so that it cannot spread and cause damage within the body, that person is considered as having resistance to the infectious agent.

Booster vaccinations are second (or subsequent) vaccinations, separated from the first or prior vaccination by certain periods (often a span of several years) that result in an upsurge in or an extension of the effectiveness of the original vaccinations. Booster vaccinations take advantage of immunologic memory. When the white blood cells that have the ability to recognize and respond to a particular antigen are exposed to that antigen as part of a booster vaccination, those abilities are strengthened and reinforced. Some vaccinations require booster doses, whereas others do not.

Vaccinations protect more people than just the individuals who receive them. They also protect unvaccinated people who live around those who have been vaccinated, a concept known as herd immunity. A vaccination protects a person from contracting and then spreading an infection. When enough people living in an area have been immunized, the relevant disease-causing agent will have difficulty spreading in that area because it cannot find new human hosts * . The disease is much less likely to be passed on, even to those few people who remain unvaccinated. It is estimated that the protection of unvaccinated people in a community occurs when the proportion of persons in that community who are vaccinated reaches 95 percent.

Vaccinations boost immunity through a so-called active process, meaning that the immune system, in responding to an antigen, is reacting actively. There is also passive immunity, which is immunity acquired through the transfer of antibodies * * and, after birth, from the mother's breast milk to the infant. A mother who has immunity against tetanus, a disease of the central nervous system caused by the toxin secreted by the tetanus bacterium, for example, will pass this immunity along to the developing fetus through the transfer of antibodies while she is pregnant. However, the offspring's immunity is only temporary. The infant's passive immunity disappears a few months after birth. Infants later make their own antibodies after being exposed to an infectious agent or receiving a vaccine.

How Successful Are Vaccines?

Vaccinations are one of the great success stories of public health programs in the 20th century. Widespread use of vaccines has brought about dramatic reductions in illness, disability, and death from many diseases, including potentially deadly and disabling childhood diseases such as diphtheria, tetanus, pertussis (whooping cough), measles, and poliomyelitis. Before measles vaccination began in the United States in 1963, for example, an estimated 500,000 cases and 500 measles-associated deaths in the United States were reported each year, according to the Centers for Disease Control and Prevention (CDC). After immunizations began, these statistics dropped sharply. The United States had been averaging from 90 to 100 cases of measles each year, but the number of cases began increasing in the 2010s, with 189 reported cases in 2015. This is due partly to children being exempt from vaccinations for religious and other personal reasons.

In the 1920s, before vaccination for diphtheria was available, approximately 150,000 cases of diphtheria occurred annually in the United States. Diphtheria is a frightening and painful disease. The thick mucus * produced in the throat and nasal passages in the course of the disease closes the airway and can lead to suffocation. After the introduction of diphtheria vaccine, millions of people were protected from this disease. Because of diphtheria vaccination, the illness became virtually nonexistent in developed nations.

Poliomyelitis, a viral disease characterized by inflammation of nerve cells in the brain and spinal cord, otherwise known as polio and infantile paralysis, was a health scourge in the United States until the 1950s. The virus can spread along nerve fiber pathways, damaging and even destroying motor neurons (especially the motor neurons that supply muscles in the legs). This damage to nerve tissue often produces muscle weakness and paralysis * . The virus may damage neurons that supply the chest-wall muscles and the diaphragm, a thin sheet of muscle separating the chest from the abdomen that is essential to breathing. Paralysis of these muscles can lead to respiratory failure * and death.

Smallpox is the only disease that has been eradicated entirely from the global population through an aggressive international immunization program. This highly contagious disease once killed as many as one out of every three infected individuals. The last case of smallpox in the United States was in 1949. The last naturally occurring case in the world was in Somalia in 1977. After the World Health Organization declared in 1980 that the disease had been eliminated from the world, routine vaccination against smallpox among the general public was stopped because it was no longer necessary for prevention. However, since smallpox is considered a weapon of bioterrorism, the U.S. government is taking precautions in the event of a smallpox outbreak.

What Vaccines Are Used in the 21st Century?

The 20th century saw both tremendous successes of vaccines and advances in vaccine technology. Some earlier vaccines were improved, and new vaccines were introduced. There are four different types of vaccines available:

  This information is not a tool for self-diagnosis or a substitute for professional care.