Explainer: What is a vaccine? | Science News for Students

Explainer: What is a vaccine?

Vaccines imitate infections to prevent illness
Jul 18, 2017 — 7:05 am EST
vaccine shot

Most vaccines are given through injections, but some can be given by nose or mouth.


Most vaccines are given through injections, but some can be given by nose or mouth.


The immune system is the body’s natural defense against germs. When a germ invades, the immune system sends special cells to fight it off. Sometimes, though, the immune system is not naturally strong enough to prevent a disease from seriously harming someone or even killing them. But there’s a way to give the immune system a boost. It’s known as a vaccine.

Vaccines are substances that prepare the immune system to fight a disease-causing germ or other pathogen by imitating an infection. They trick the immune system into making a “memory” of that germ without ever having to fight the real germ in the first place. Now, when the immune system encounters the real pathogen — whether it’s a virus, bacterium or other microbe — it is ready to attack it. As a result, the vaccinated person doesn’t get sick.

Before vaccines, many children suffered from debilitating — and killer — illnesses such as measles, polio, smallpox and diphtheria. A simple scratch could be deadly if it became infected with the bacteria responsible for tetanus (or “lockjaw”). Vaccines, though, have changed this. Smallpox is completely gone from the world, and polio nearly so. Outbreaks of measles and diphtheria are rare, especially in the United States. Tetanus infections continue to decline worldwide.

Vaccinations also can help protect people who can’t be vaccinated. These might be babies who are too young to receive a vaccine. Or there might be people who are too ill or too old to be vaccinated. When enough people in a community are vaccinated against a particular contagious disease, there’s little opportunity for that disease to spread from person to person. Doctors call this type of community protection “herd immunity.”

How vaccines work

The body is designed to fight off foreign invaders, such as germs. To do this, it needs to be able to distinguish elements of itself from outside agents (nonself). This is the immune system’s job — to recognize self from nonself. To achieve this, the immune system is always on the lookout for evidence of foreign stuff, or what scientists refer to as antigens.

antibody antigen
Antibodies are Y-shaped proteins. They bind to disease-causing antigens. Each antibody binds a specific antigen. The two fit together like a lock and key.
Fvasconcellos/Wikimedia Commons

Pathogens are covered with tiny particles. Since they look unfamiliar to the human body, the immune system calls out its antigen attack squads. Those antigens are specific to a particular germ. A flu virus and a measles virus, for instance, have different antigens. Even two different types of flu viruses may contain somewhat different antigens.

So when a pathogen enters the body, the immune system dispatches special molecules — antibodies. These Y-shaped proteins patrol for particular antigens. When they find the ones they are trained to deal with, the antibodies go to work. They have two jobs. One is to disarm the threat by locking onto the antigens. This stops the germs from doing further harm. The antibodies also signal other immune cells to come to the site of the infection. Those other immune cells destroy the germs and remove them from the body. This entire process can take several days from the first time the body encounters a new type of germ.

Once an infection goes away, however, it won’t be forgotten. The immune system has stored a “memory” of the pathogen’s antigens in what are called B cells. These memory cells make antibodies designed to recognize and lock up the antigens for that specific germ. So if the same germ enters the body again, these new antibodies can now recognize it right away. They can direct the immune system to destroy and remove the germ from the body before it multiplies and forms an infection.

This protection against previous illnesses is called immunity. And it’s why a child who had chickenpox as a child won’t ever come down with that illness again.

One way to build immunity is through infection. A person who survives Ebola, for instance, will never suffer through that disease again. But since infection can damage or kill someone, the second way to build immunity — with a vaccine — is the safer option when it is available. (Scientists haven’t yet developed vaccines against every disease. Ebola is one disease for which no vaccine yet exists.)

Some vaccines are made from a weakened form of a disease-causing germ. Others are made from dead or deactivated forms of the germ. Most vaccines are given by injection. Some can be given by mouth or as a nasal spray.

The vaccine won’t cause disease, because the weakened germ can no longer copy itself to release new germs inside the body. The immune system, though, doesn’t know that the germ’s mimic (or vaccine) is weakened or dead. All it sees are the antigens signaling a now-familiar invader. It sends antibodies and other immune cells to stop and destroy the invader. The antibodies “remember” the germ. This creates immunity against future infections with the same type of germ.

Some antibodies protect for a lifetime. Other times, the immune system’s memory of a germ can fade over time. When this happens, immunity can falter and leave a person vulnerable to infection. In these cases, the immune system needs a reminder. These reminders are called boosters. Tetanus is an example. Doctors recommend a tetanus booster every 10 years to maintain good immunity against the disease.

Some germs also change substantially over time — evolve — creating new antigens. Influenza viruses are well known for doing this. Now the old antibodies may no longer recognize the new form of the germ (and antigens) and therefore fail to protect against it. That’s why flu vaccines must be given every year. Each new vaccine deals with the latest versions of these ever-changing viruses.

Power Words

antibody    Any of a large number of proteins that the body produces from B cells and releases into the blood supply as part of its immune response. The production of antibodies is triggered when the body encounters an antigen, some foreign material. Antibodies then lock onto antigens as a first step in disabling the germs or other foreign substances that were the source of those antigens. 

antigen     A substance capable of triggering an immune reaction.

bacterium     ( plural bacteria ) A single-celled organism. These dwell nearly everywhere on Earth, from the bottom of the sea to inside of plants and animals.

B cell     A type of small white blood cell (also known as a B lymphocyte), which plays an important role in the immune system. Made in the bone marrow, these cells mature into plasma cells, and serve as the source of antibodies.

cell     The smallest structural and functional unit of an organism. Typically too small to see with the naked eye, it consists of watery fluid surrounded by a membrane or wall. Animals are made of anywhere from thousands to trillions of cells, depending on their size. Some organisms, such as yeasts, molds, bacteria and some algae, are composed of only one cell.

contagious     An adjective for some disease that can be spread by direct contact with an infected individual or the germs that they shed into the air, their clothes or their environment. Such diseases are referred to as contagious.

Ebola     A family of viruses that cause a deadly disease in people. All cases have originated in Africa. Its symptoms include headaches, fever, muscle pain and extensive bleeding. The infection spreads from person to person (or animal to some person) through contact with infected body fluids. The disease gets its name from where the infection was first discovered in 1976 — communities near the Ebola River in what was then known as Zaire (and is now the Democratic Republic of Congo).

germ     Any one-celled microorganism, such as a bacterium, fungal species or virus particle. Some germs cause disease. Others can promote the health of higher-order organisms, including birds and mammals. The health effects of most germs, however, remain unknown.

herd immunity     A term that refers to the idea that if most people within a population are immune to a disease, then they can’t spread it. This tends to protect most people who are not immune.

immune     Able to ward off a particular infection.

immune system     The collection of cells and their responses that help the body fight off infections and deal with foreign substances that may provoke allergies.

immunity     The ability of an organism to resist a particular infection or poison by

infection     A disease that can spread from one organism to another. It’s usually caused by some sort of germ.

influenza    (also known as flu) A highly contagious viral infection of the respiratory passages causing fever and severe aching. It often occurs as an epidemic.

measles     A highly contagious disease, typically striking children. Symptoms include a characteristic rash across the body, headaches, runny nose, and coughing. Some people also develop pinkeye, a swelling of the brain (which can cause brain damage) and pneumonia. Both of the latter two complications can lead to death. Fortunately, since the middle 1960s there has been a vaccine to dramatically cut the risk of infection.

microbe     Short for microorganism. A living thing that is too small to see with the unaided eye, including bacteria, some fungi and many other organisms such as amoebas. Most consist of a single cell.

molecule     An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).

nasal     Having to do with the nose.

outbreak     The sudden emergence of disease in a population of people or animals. The term may also be applied to the sudden emergence of devastating natural phenomena, such as earthquakes or tornadoes.

particle     A minute amount of something.

pathogen     An organism that causes disease.

polio     An infectious viral disease that affects the central nervous system and can cause temporary or permanent paralysis.

protein     Compound made from one or more long chains of amino acids. Proteins are an essential part of all living organisms. They form the basis of living cells, muscle and tissues; they also do the work inside of cells. The antibodies that attempt to fight infections are among the better-known, stand-alone proteins.

tetanus     An unusual type of bacterial infection that usually does not spread between infected individuals. The source of these germs is typically exposure to soil, dust or animal feces. The disease causes stiff, painful muscles throughout the body, including a cramping in the jaw (which is why it’s sometimes called “lockjaw”). Fevers, seizures, sweating and headaches may also occur.

vaccine    (v. vaccinate) A biological mixture that resembles a disease-causing agent. It is given to help the body create immunity to a particular disease. The injections used to administer most vaccines are known as vaccinations.

virus     Tiny infectious particles consisting of RNA or DNA surrounded by protein. Viruses can reproduce only by injecting their genetic material into the cells of living creatures. Although scientists frequently refer to viruses as live or dead, in fact no virus is truly alive. It doesn’t eat like animals do, or make its own food the way plants do. It must hijack the cellular machinery of a living cell in order to survive.