How Vaccines Prevent Disease

A medical professional prepares an immunization injection. Photo Credit

Our bodies have several lines of defense against infections and disease. Our immune system is made up of cells, tissues, and organs working together to fight off infections. Specialized cells form antibodies in response to pathogen antigens; other cells engulf microbes.

Immune system mechanisms fight pathogens that have already invaded your body. Vaccines are able to mimic pathogen antigens in order to form protection against diseases before you actually get them. When you are later exposed to the disease, your body already has defenses in place to fight the disease, and you don’t get sick.



active immunity – resistance to disease acquired by either exposure to a pathogen or by receiving a vaccine against a pathogen.

antibodies – chemicals made by B lymphocytes that can bind to antigens.

antigen – any type of molecular structure that lymphocytes recognize as foreign to the body and that can trigger an immune response.

B cells – a lymphocyte that produces antibodies that bind to an antigen.

disease – a condition in which the body cannot function normally due to infection by a pathogenic agent, genetic condition, nutritional deficiency, or an illness of an affected body organ.

herd immunity – resistance to an infectious agent by an entire group or community; a large number of immune persons in a community reduces the likelihood that an infected person will come into contact with a susceptible person among the population.

immune system – the body system that is involved in producing defenses against infectious organisms.

immunity – resistance to disease.

infection – a condition in which an infectious agent invades the body and begins to multiply.

natural immunity – active immunity that is produced from the body’s response to a pathogen.

passive immunity – immunity that is produced from the body’s response to antibodies that have not been produced by the body’s own immune system.

pathogen – an organism that causes disease.

vaccine – an antigen containing substance that stimulates the production of antibodies.

vaccine-induced immunity – immunity resulting from receiving a vaccine.


Medical vials filled with vaccine and a syringe. Photo Credit

Vaccines take advantage of the body’s natural abilities to fight disease. Just as the body produces antibodies that react to invading microorganisms, vaccines are designed to boost the immune system’s ability to quickly recognize microbes and destroy them before the disease process begins.

Most vaccines contain killed or weakened versions of a specific microbe or parts of a microbe. They cannot cause disease, but still produce an immune response. When the dead or weakened microbes are introduced to the body through vaccination, the body responds by destroying the microbe and also by producing a “memory” of the microbe in the immune response system. If the body is later exposed to the disease microbe, the immune system responds much more quickly to eliminate it because it has already formed antibodies.

This is active immunity. Active immunity results after exposure to a pathogen produces an immune response. Exposure to the actual disease is called natural immunity. Immunity from exposure to a vaccine is called vaccine-induced immunity. Active immunity usually lasts for years and often if a person has had the disease, they will not get that disease again because the antibodies remain in the blood, providing lasting protection.

In vaccine-induced immunity, your body produced antibodies in response to a vaccine; you don’t have to get sick. The length of time for active immunity protection varies with the type of disease.


Why do you get a flu shot every year if active immunity is long-lasting?

Flu viral strains change from year to year. Scientists research which strains are most likely to be circulating in the population each year and produce a vaccine to protect against the most prevalent strains. The flu shot you get is a different one each year.


Some diseases require booster shots to continue immunity protection.

Vaccines are specific to the disease for which they are produced. For example, tetanus vaccines only prevent tetanus; mumps vaccines only prevent mumps.

Another type of immunity, passive immunity, results from being given antibodies which have been produced by another person or animal. Passive immunity still results in an immune response, but is not long-lasting because the person’s immune system has not engaged the cells necessary to produce a long defense.


How does a vaccine produce an immune response?

Vaccines trigger the immune system’s disease fighting cells to produce antibodies. White blood cells are the body’s disease-fighting cells. Some white blood cells (leukocytes) circulate through the body, encounter diseased, damaged, or dead cells and destroy them. Other white blood cells (lymphocytes), provide protection against specific diseases. The most important types of lymphocytes are B cells and T cells.

B Cells

B cells make antibodies after being exposed to specific antigens (a substance that causes an immune response.) Most vaccines work by stimulating B cells to produce antibodies and a “memory response” mechanism that allows the body to recognize a disease-producing organism the next time it comes into contact with it. The antibody binds to the microbe and blocks its ability to cause disease.


T Cells

Scanning electron micrograph of a red blood cell (left), a platelet (center) and a T cell (right). Credit: Electron Microscopy Facility at the National Cancer Institute at Frederick.

T cells, known as killer T cells, destroy infected or abnormal cells by releasing toxic chemicals. Other types of white blood cells help B and T cells in the immune response. Helper T cells and dendritic cells are two examples.


The Importance of Vaccines

Vaccines have saved millions of lives. Diseases that once ravaged populations are now preventable by vaccines. Smallpox, for example, was eradicated worldwide.

A man showing the effects of polio. Content: CDC/NIP/Barbara Rice.
Photo Credit

Measles can be almost completely prevented by the vaccine, but outbreaks of measles have occurred when unvaccinated people become infected and spread the disease throughout a population to susceptible individuals. Several outbreaks of measles have occurred in recent years. Officials at the World Health Organization said they were “taken aback at the than 22,000 cases in 2014.” Recurring measles outbreaks over the globe threaten the goal of eliminating measles from many countries.

Polio, a disease that resulted in millions of cases of paralysis, has been greatly reduced or eliminated in many countries, although it still occurs in some areas of the world. There is no treatment or cure for polio once a person is infected, but with supportive care, polio victims may survive. Polio is completely preventable by receiving the polio vaccine in the required number of doses.

Infant vaccination against Haemophilus influenza type b (Hib) has greatly reduced the cases of infant meningitis. About 1 in 200 children under 5 years got meningitis before the vaccine was developed. Meningitis killed many children and left survivors with permanent brain damage. After the vaccine, the incidence of Hib dropped by 99%.


Herd Immunity

Another way vaccines prevent disease is through herd immunity. Herd immunity is the resistance to an infectious agent by an entire group or community. It results when a large number of vaccinated persons in a community reduces the likelihood that an infected person will come into contact with a susceptible person among the population.

If a large enough segment of the population is immunized against a certain disease, it is unlikely that an unvaccinated person will come into contact with a person infected with the pathogen; the pathogen does not survive within the population. Therefore, the disease does not spread through the population.

Some people cannot be immunized. Small children who are not yet old enough to receive certain vaccinations are vulnerable to diseases until they receive the vaccine. Other children and some adults have chronic medical conditions which prohibit them from receiving vaccinations. Pregnant women may not be eligible for certain vaccines. These children and adults are protected by herd immunity.


Vaccines and Adverse Reactions

Vaccines are developed according to strict guidelines, procedures, and testing protocols with the highest safety standards. Years of testing occur before vaccines are approved and distributed.

However, a few people have different reactions to vaccines. All medicines have potential side effects. Most adverse reactions are short-lived and are just an immune response to receiving dead or weakened pathogen particles. Having the disease is usually a much more serious condition with potentially longer lasting consequences.

In recent years, controversy about vaccine safety has caused some people to stop or delay immunizing their children or themselves. Scientific evidence has disproven the unfounded links between vaccines and adverse health conditions. Relying on the evidence is key to making responsible health decisions, including those decisions about immunizations.