Vaccination, also called immunization, is the administration of a biological produce, often a weakened (attenuated) or killed microorganism to stimulate immune system defenses against that organism.

A baby receiving a vaccine injection.

A baby receiving a vaccine injection.
(CDC/Amanda Mills)

Upon future exposure to the pathogen, the immune system responds rapidly and prevents the development of disease.


Vaccinations are among the most effective public health interventions for preventing the spread of contagious diseases. Vaccination has changed the course of human history, putting an end to plagues of infectious diseases that once wiped out large segments of the population. Vaccination has led to the worldwide eradication of smallpox and the near-eradication of poliomyelitis. In many parts of the world, vaccination has nearly eliminated diseases such as measles, mumps, rubella, and chickenpox (varicella), which were once considered to be a normal part of childhood, but sometimes led to debilitating or fatal complications. Vaccination has also drastically reduced the incidence of diseases such as tuberculosis, whooping cough (pertussis), rabies, diphtheria, typhoid fever, tetanus, and plague. More recent vaccines have decreased the incidence of hepatitis B infections that can lead to liver cancer and human papilloma virus (HPV) infections that can lead to cervical cancer. Other new vaccines have been developed against Ebola virus disease and dengue fever, as well as a better vaccine against shingles. As of 2018, there were vaccines available against more than 33 infectious diseases.

In the United States and other developed countries, children are routinely vaccinated against more than 12 diseases. In the United States, they must have at least some of these vaccinations before they can attend school. As a result, these diseases are now at their lowest levels in human history. Once most people in a population have been vaccinated against an infectious disease, it can no longer spread through the population. This is known as herd immunity, and provides protection against the disease, even for those who have not been vaccinated or who cannot be vaccinated for medical reasons.


Albert Bruce Sabin was born on August 26, 1906, in Bialystok, Russia, to Jacob and Tillie Sabin. To escape extreme poverty, the Sabins immigrated to the United States and settled in Paterson, New Jersey. Following his graduation from high school in 1923, Sabinwas able to attend dentistry school at New York University due to his uncle's generous financing. However, after reading Paul deKruif's Microbe Hunters, he became intrigued by virology and the idea of curing epidemic diseases. After two years of dentistry school, Sabin decided to switch to medicine, earning his M.D. in 1931. Sabin completed his residency and internship in the United States and then went to London to conduct research.

Sabin returned to the United States in 1935 to resume his research on polio at the Rockefeller Institute. In 1953, Jonas Salk announced that he had created a safe killed-virus polio vaccine, but soon after its introduction many people died. Sabin wanted to create a live-virus vaccine, which he believed would be safer. Sabin diluted three strains of the polio virus and tested these on himself, his family, and other volunteers. These live-virus vaccines (given orally) proved safe and effective and soon became the vaccine of choice worldwide. Sabin's published works include Viruses and Cancer: A Public Lecture in Conversational Style (1965), Behavior of Chimpanzee-Avirulent Poliomyelitis Viruses in Experimentally Infected Human Volunteers (1955), and Recent Advances in Our Knowledge of Dengue and Sand Fly Fever (1955). Sabin died of congestive heart failure on March 3, 1993.


Many vaccines are produced from weakened or killed bacteria or viruses. Vaccination stimulates the body's immune system to respond to antigens or markers on the pathogen by producing proteins called antibodies that are specific for those antigens. Newer vaccines are made using genetic engineering. Called subunit vaccines, they use only a few of the a pathogen's antigens to stimulate immunity. After vaccination, when people are exposed to the infectious microorganism, they already have antibodies to destroy it, thereby preventing the development of disease. This process of establishing specific immunity through vaccination is known as immunization.

It has been recognized for hundreds of years that people who survived an infectious disease were very unlikely to contract that disease a second time. During the eighteenth century, smallpox was responsible for 60 million deaths worldwide,—killing 45,000 people annually in the British Isles alone. Early in that century, Lady Mary Wortley Montagu observed the practice of variolation, a predecessor to vaccination, in Turkey. Every autumn, old women would collect pus from the victim of a mild smallpox attack and inject under the skin of susceptible children. After a short illness, the children recovered and were thereafter immune to smallpox. This worked because smallpox could be caused by two variant viruses, one that caused relatively mild illness and the other major illness. Montagu popularized variolation among the British gentry. However, treatment with live virus sometimes resulted in disease transmission or death. Dr. Edward Jenner, adopting a related method used by some English farmers, developed the first vaccination against smallpox using the pus from cowpox, a closely related cattle disease that does not cause human illness.

Vaccinations vary for different diseases. Some vaccinations, such as the rabies vaccine first developed by Louis Pasteur and Emile Roux in 1885, are administered only when a person is likely to have been exposed to the rabies or is at specific risk (e.g., veterinarians). Other vaccines are given to travelers going to countries where certain diseases, such as typhoid or yellow fever, are common or endemic. Influenza vaccines are developed annually, based on the viral strains that are expected to be prevalent that year because the influenza virus mutates rapidly and easily. Although annual flu shots were previously recommended only for specific groups who were particularly susceptible to complications—such as young children, the elderly, and people with compromised immune systems—they are now recommended for everyone over age six months. Attempts are also made to vaccinate almost everyone against certain other diseases, such as diphtheria, tetanus, polio, and measles. Some vaccines are required to be given only once, whereas as others require a series of shots or must be repeated after a certain number of years. Most vaccines are given as injections, but a few are given by mouth (oral). Some vaccines are combined in one injection, such as the measles-mumps-rubella (MMR) or diphtheria-tetanus-pertussis (DTaP) combinations.

Recommended vaccinations

The 10 countries with the most under-and un-vaccinated children (based on coverage with the first and third doses of DTP containing vaccines), 2015.

The 10 countries with the most under-and un-vaccinated children (based on coverage with the first and third doses of DTP containing vaccines), 2015.

Since most vaccines have only become available in recent decades, older adults are presumed to be immune to most common infectious diseases through previous exposure. For example, adults born before 1957 do not need measles vaccinations, because they have lived through several years of measles epidemics, and 95–97% are immune to measles. Recommendations for adult vaccinations for those not already immune include:

Other vaccines available in the United States on an as-needed basis protect against:

Recommended vaccine dosages depend on the type of vaccine and may be different for different patients and different ages. For example, some vaccines, such as MMR and MMRV, are not given during the first year, when babies are still protected by their mothers' antibodies that can destroy the vaccine. Most physicians follow recommended schedules, but there is some flexibility. For example, children are usually not vaccinated during an illness. An immunization record is used to keep track of children's vaccinations.

Some people including healthcare workers, college students, and travelers to developing countries may require additional vaccinations, some of which must be given as far as 12 weeks in advance. Women of childbearing age may also require additional vaccinations, since some infectious diseases are very dangerous if contracted during pregnancy, and some vaccines cannot be administered during pregnancy.


Some vaccinations are not recommended for pregnant or breastfeeding women. Others may be given to pregnant women who are at especially high risk for a disease such as polio or to prevent medical problems in their babies. Women should avoid becoming pregnant within three months of vaccination against measles, mumps, or rubella, as these vaccines may harm the fetus. Certain vaccinations are not safe for infants.

Medical conditions, such as cancer, HIV/AIDS, or inherited immune deficiency diseases can render vaccinations dangerous. Influenza vaccine may reactivate the rare Guillain-Barrésyndrome or worsen illnesses that involve the lungs, such as bronchitis or pneumonia. Vaccines with fever as a side effect may trigger seizures in people, especially young children with a history of fever-induced seizures. These seizures cause no permanent harm but are frightening.

Vaccines can cause allergic reactions in some people. People who have previously had an unusual reaction to a vaccine should inform their physician. Patients with allergies to:

An infectious bacterial disease of animals that can be passed to humans.
Proteins produced by specialized white blood cells after stimulation by a foreign substance (antigen), including a vaccine, that act against the antigen in an immune response.
Any foreign substance, usually a protein, that stimulates the body's immune system to produce antibodies.
A vaccine made with live virus that has been modified so that it is no longer infective.
A variable developmental disorder that includes an impaired ability to communicate and form normal social relationships.
Booster shot—
A supplementary vaccine dose.
Cellular immune system—
Cell-mediated immunity; immune system cells that fight infection without specific antibodies against the disease-causing organism.
A bacterial infection of the small intestine that spreads through feces-contaminated water and food and is often fatal in young children and the elderly; cholera vaccinations are essential in the overcrowded, unsanitary conditions of many refugee camps.
A mild disease in cows caused by a virus related to the smallpox virus.
A serious, infectious disease caused by a toxin-producing bacterium.
DTap, TDap—
Combination vaccines against diphtheria, tetanus, and pertussis; Td is a tetanus/diphtheria vaccine.
Occurring naturally and consistently in a particular area.
An outbreak of disease where the number of cases exceeds the usual (endemic) or typical number of cases.
Any of several viral diseases that affect the liver; vaccinations (HepA and HepB) are available against hepatitis A. B, and C.
Herd immunity—
Disease protection for nonimmune or unvaccinated individuals that is conferred by the prevailing immunity within a population due to widespread vaccination coverage.
A vaccine against Haemophilus influenzae type b, a major cause of spinal meningitis.
Human papillomavirus; HPV—
A large family of viruses that cause warts and other growths on various parts of the body; the HPV vaccine is effective against sexually transmitted HPV strains that cause cervical cancer.
The administration of a vaccine that stimulates the body to create antibodies to a specific disease (immunity) without causing symptoms of the disease.
An acute and highly contagious viral disease marked by distinct red spots followed by a rash; occurs primarily in children.
Inflammation of tissues that surround the brain and spinal cord; the meningococcal conjugate vaccine (MCV) protects against meningococcal meningitis.
A combination vaccine against measles, mumps, and rubella.
A combination vaccine against measles, mumps, rubella, and varicella (chickenpox).
An acute and highly contagious viral illness that usually occurs in childhood.
A disease-causing microorganism.
A vaccine against the pneumococcus that causes bacterial pneumonia.
Pertussis; whooping cough—
An infectious bacterial disease, especially of children, that is on the increase due to under-vaccination.
A highly infectious and often fatal bacterial disease that is transmitted between humans and from rodents to humans by fleas.
Poliomyelitis; polio—
An infectious viral disease that was very widespread, especially among children, before the introduction of polio vaccines in the 1950s; it has been eradicated in most of the world through vaccines such as inactivated polio vaccine (IPV).
A rare but serious viral disease transmitted to humans by the bite of an infected animal.
Rotavirus (RV)—
A genus of viruses, some of which are common causes of diarrhea and childhood death worldwide.
German measles; a contagious viral disease that is milder than typical measles but is damaging to the fetus if it is contracted in early pregnancy.
A highly contagious viral disease that has been eradicated through worldwide vaccination.
An acute infectious disease caused by bacteria introduced into the body through a wound.
A chronic infectious bacterial disease of the respiratory system.
Typhoid fever—
An infectious bacterial disease spread through poor sanitation.
A preparation of killed or attenuated-live bacteria or virus that is administered to stimulate immunity against that disease-causing organism.
Varicella; chickenpox—
A childhood disease caused by a herpes virus that may be reactivated later in life to cause herpes zoster or shingles.
The deliberate inoculation of an uninfected person with smallpox to protect against severe smallpox; widely used before the introduction of a vaccine.
Yellow fever—
An infectious viral disease that is spread by mosquitoes and is most common in Central and South America and Central Africa.

Side effects

The most common vaccination side effects are minor pain, redness, or swelling at the site of the injection. Some people may develop a fever or rash. In rare cases, vaccines may cause severe allergic reactions. The Vaccine Adverse Event Reporting System (VAERS) is a national system for tracking side effects and adverse reactions to any vaccine licensed in the United States. Anyone, including patients, can report an adverse reaction to this system.



Vaccines may interact with other medicines and medical treatments to alter the effects of the vaccine or the other medication or increase the risk of side effects. For example, radiation therapy and cancer drugs may reduce the effectiveness of many vaccines or increase the chance of side effects.

Public health role and response

Vaccinations are a major public health concern worldwide. Vaccinations are essential for preventing or containing epidemics of infectious disease following natural and manmade disasters. Overcrowded refugee camps, where diseases such as cholera and measles can reach epidemic proportions, are a special focus for mass vaccinations.

Both the Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO) carefully track vaccination coverage throughout the world. Routine childhood vaccinations and mass immunization campaigns are primary public health strategies for both organizations, with the goals of reducing mortality from vaccination-preventable diseases and eventual worldwide eradication of those diseases. The WHO's fourth Millennium Development Goal was to reduce the mortality rate for children under five by two-thirds between 1990 and 2015. Measles vaccination coverage is an indicator of progress toward that goal because it is considered an indicator of access to child health services. The Measles & Rubella (MR) Initiative is a joint effort of WHO, the CDC, UNICEF, the United Nations Foundation, and the American Red Cross. It aims to reduce global measles deaths by at least 95% from 2000 levels and to eliminate measles and rubella in at least five WHO regions by 2020. Polio has been the focus of the Global Action Vaccine Plan. As of early 2018, polio was close to eradication with mild cases found only in Pakistan and Nigeria, although half a dozen other countries were considered at high risk for resurgence of the disease.

Mass vaccination programs face a variety of obstacles around the world, ranging from lack of resources and healthcare infrastructures to cultural and political opposition. For example, WHO healthcare workers administering polio vaccine in Pakistan have been assassinated amid rumors that the polio vaccine was unsafe and an element in a plot to sterilize Muslim children.

In the United States, falling vaccination rates are a major public health concern and have led to serious outbreaks of pertussis, and measles. Overall vaccination rates in some parts of the Pacific Northwest and a few other communities have fallen below 80%, well below the threshold needed for 93–95% needed for herd immunity. In some Oregon communities, more than 20% of children have religious exemptions from required vaccinations. Other parents delay vaccinations for their young children because of misinformed fears about the dangers of vaccines. Whereas in the developing world, poverty and poor healthcare infrastructure are common barriers to vaccination, in the United States, most low-income children receive the recommended vaccinations through the federal Vaccines for Children program, which provides free vaccinations for children without health insurance. In contrast, it tends to be wealthier, better-educated American parents who object to vaccinating their children.

The development of new vaccines, especially for widespread infections such as malaria and HIV/AIDS, is also a major public health initiative. New vaccines for rare infectious diseases, such as Ebola and Zika virus are being developed and tested for defense against potential bioterrorism.

See also Centers for Disease Control and Prevention ; Diptheria ; Ebola virus disease ; Endemic ; Flu pandemic ; Food and Drug Administration ; Measles ; Mumps ; Smallpox ; Whooping cough ; Zika virus disease .



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Children's Hospital of Philadelphia Vaccine Education Center, 3401 Civic Center Blvd., Philadelphia, PA, 19104, (800) 879-2467, .

National Institute of Allergy and Infectious Diseases Office of Communications and Government Relations, 6501 Fisher Lane, MSC 9806, Bethesda, MD, 20892-9806, (301) 496-5717, (866) 284-4107 or TDD: (800)877-8339, Fax: (301) 402-3573,, .

United States Centers for Disease Control and Prevention (CDC), 1600 Clifton Road, Atlanta, GA, 30329-4027, (404) 639-3534, (800) CDC-INFO (800-232-4636); TTY: 888-232-6348, .

World Health Organization (WHO), Avenue Appia 20, 1211 Geneva 27, Switzerland, +22 41 791 21 11, Fax: +22 41 791 31 11,, .

Larry I. Lutwick, MD
Margaret Alic, PhD
Revised by Tish Davidson, AM

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