Understanding How Vaccines Work

Understanding mRNA COVID-19 Vaccines

mRNA vaccines are a new type of vaccine to protect against infectious diseases. Learn about how COVID-19 mRNA vaccines work.

The Immune System—The Body’s Defense Against Infection

To understand how vaccines work, it helps to first look at how the body fights illness. When germs, such as bacteria or viruses, invade the body, they attack and multiply. This invasion, called an infection, is what causes disease. The immune system uses your white blood cells to fight infection. These white blood cells consist primarily of macrophages, B-lymphocytes and T-lymphocytes:

  • Macrophagesmedia icon are white blood cells that swallow up and digest germs, plus dead or dying cells. The macrophages leave behind parts of the invading germs called antigens. The body identifies antigens as dangerous and stimulates antibodies to attack them.
  • B-lymphocytes are defensive white blood cells; they can produce antibodies to fight off infection.
  • T-lymphocytes are another type of defensive white blood cell, that recognizes a familiar germ, if the body is exposed again to the same disease

The first time the body is infected with a certain germ, it can take several days for the immune system to make and use all the tools needed to fight the infection. After the infection, the immune system remembers what it learned about how to protect the body against that disease. If your body encounters the same germ again, the T-lymphocytes recognize the familiar germ and the B-lymphocytes can produce antibodies to fight off infection.

How Vaccines Work

Vaccines can help protect against certain diseases by imitating an infection. This type of imitation infection, helps teach the immune system how to fight off a future infection. Sometimes, after getting a vaccine, the imitation infection can cause minor symptoms, such as fever. Such minor symptoms are normal and should be expected as the body builds immunity.

Once the vaccinated body is left with a supply of T-lymphocytes and B-lymphocytes that will remember how to fight that disease. However, it typically takes a few weeks for the body to produce T-lymphocytes and B-lymphocytes after vaccination. Therefore, it is possible that a person infected with a disease just before or just after vaccination could develop symptoms and get that disease, because the vaccine has not had enough time to provide protection. While vaccines are the safest way to protect a person from a disease, no vaccine is perfect. It is possible to get a disease even when vaccinated, but the person is less likely to become seriously ill.

Types of Vaccines

Scientists take many approaches to developing vaccines. These approaches are based on information about the diseases the vaccine will prevent, such as how germs infect cells, how the immune system responds to it, regions of the world where the vaccine would be used, the strain of a virus or bacteria and environmental conditions. Today there are five main types of vaccines that infants and young children receive in the U.S.:

  • Live, attenuated vaccines fight viruses and bacteria. These vaccines contain a version of the living virus or bacteria that has been weakened so that it does not cause serious disease in people with healthy immune systems. Because live, attenuated vaccines are the closest thing to a natural infection, they are good teachers for the immune system. Examples of live, attenuated vaccines include measles, mumps, and rubella vaccine (MMR) and varicella (chickenpox) vaccine. Even though they are very effective, not everyone can receive these vaccines. Children with weakened immune systems—for example, those who are undergoing chemotherapy—cannot get live vaccines.
  • Non-live vaccines also fight viruses and bacteria. These vaccines are made by inactivating, or killing, the germ during the process of making the vaccine. The inactivated polio vaccine is an example of this type of vaccine. Often, multiple doses are necessary to build up and/or maintain immunity.
  • Toxoid vaccines prevent diseases caused by bacteria that produce toxins (poisons) in the body. In the process of making these vaccines, the toxins are weakened so they cannot cause illness. Weakened toxins are called toxoids. When the immune system receives a vaccine containing a toxoid, it learns how to fight off the natural toxin. The DTaP vaccine contains diphtheria and tetanus toxoids.
  • Subunit vaccines include only parts of the virus or bacteria, or subunits, instead of the entire germ. Because these vaccines contain only the essential antigens and not all the other molecules that make up the germ, side effects are less common. The pertussis (whooping cough) component of the DTaP vaccine is an example of a subunit vaccine.
  • Conjugate vaccines fight a type of bacteria that has antigens. These bacteria have antigens with an outer coating of sugar-like substances called polysaccharides. This type of coating disguises the antigen, making it hard for a young child’s immature immune system to recognize it and respond to it. Conjugate vaccines are effective for these types of bacteria because they connect (or conjugate) the polysaccharides to antigens that the immune system responds to very well. This linkage helps the immature immune system react to the coating and develop an immune response. An example of this type of vaccine would be the Haemophilus influenzae type B (Hib) vaccine and the mRNA COVID-19 vaccine.

Vaccines Require More Than One Dose

There are four reasons that babies—and even teens or adults—who receive a vaccine for the first time may need more than one dose:

  • For some vaccines (primarily non-live vaccines), the first dose does not provide as much protection as possible. So, more than one dose is needed to build more complete immunity. For example, the Hib vaccine that protects young children against meningitis, requires 2- or 3-doses depending on manufacturer.
  • For some vaccines, protection begins to wear off over time. At that point, a “booster” dose is needed to bring protection levels back up. This booster dose usually occurs several years after the initial series of vaccine doses is given. For example, the DTaP vaccine requires the initial series of four shots for an infant to build protection against diphtheria, tetanus and pertussis. But a booster dose is needed at 4 years through 6 years old. Another booster against these diseases is needed at 11 years or 12 years of age. This booster is called Tdap.
  • For some vaccines (primarily live vaccines), more than one dose is needed for everyone to develop the best protection. For example, after one dose of the measles, mumps, and rubella vaccine, some people may not develop enough antibodies to fight off infection. The second dose helps make sure that almost everyone is protected.
  • Finally, in the case of flu vaccines, everyone 6 months and older needs to get a dose every year because each year different flu viruses can be circulating and protection from a flu vaccine wears off with time. Children 6 months through 8 years old will need to get 2 doses of vaccine the first time they get flu vaccine, or if they only got one dose in previous years, they should get 2 doses during the current year.

The Bottom Line

Vaccines are the safest way to protect against certain diseases and prevent serious illness. Natural infections can cause severe complications and be deadly. This is true even for diseases that many people consider mild, like chickenpox. It is impossible to predict who will get serious infections that may lead to hospitalization or even death. That is why vaccines are the best way to protect your child. Talk to your child’s provider to make sure they are up to date on all recommended vaccines.

Adapted from the National Institute of Allergy and Infectious Diseasesexternal icon.

For more information on vaccines call 800-CDC-INFO (800-232-4636) or visit https://www.cdc.gov/vaccines.

Page last reviewed: May 23, 2022