How Do Vaccines Work?

Vaccines work by helping people safely imitate natural infections. That means that vaccines help protect you from diseases without ever having to risk the serious, and sometimes deadly, consequences of getting sick from those diseases.

Viruses/Bacteria and Your Immune System

When viruses or bacteria (germs) invade your body, they attack and multiply. This invasion is called an infection, and the infection is what causes illness. The first time your body encounters a germ, it can take several days for your body to make and use all the tools it needs to fight the infection. After the infection is over, your body’s immune system keeps a few “memory cells” that remember what it learned about how to protect against that disease, and if your body encounters the same virus or bacteria again, it will produce antibodies to attack the germ and protect you from the disease.

How Vaccines Work

Vaccines help people develop immunity (protection) to a disease by safely imitating a natural infection. Some people may believe that natural immunity (which occurs after a person is infected by a bacteria or virus) is better than the immunity developed from vaccines. However, natural infections are dangerous because they can cause severe illness and lead to serious complications and even death.

Vaccines are made up of viruses or bacteria that are altered or weakened so that they only cause an imitation of the disease and not the disease itself.  There are a variety of different ways to alter or weaken the viruses or bacteria in vaccines so they cause immunity instead of serious disease. Click below to learn more about the different types of vaccines. They include live attenuated vaccines, toxoid vaccines, inactivated vaccines, subunit vaccines and conjugate vaccines.

To see an example of how vaccines work, read the story of Chip and Dale from the Vaccine Education Center at Children’s Hospital of Philadelphia (CHOP).

You can also view CDC’s animated video that helps explain how vaccines help babies fight infections.

Live, attenuated vaccines like MMR (measles, mumps, rubella), chickenpox and flu nasal spray (LAIV) vaccines weaken the living viruses in the vaccine so they cannot cause disease in people. Since these types of vaccines are the closest to natural infections, they are very effective, but not everyone can get them. For example, people with weakened immune systems like those undergoing chemotherapy, can’t get live vaccines.

Toxoid vaccines prevent diseases caused by bacteria that produce toxins (poisons) in the body. Like in live, attenuated vaccines, the toxins are weakened so they cannot cause illness. Weakened toxins are called toxoids. For example, DTaP and Tdap vaccines contains diphtheria and tetanus toxoids, in addition to protection against pertussis.

In inactivated vaccines (like many of the flu shots), the viruses are inactivated (killed) when making the vaccine. By killing the viruses, the vaccines produce immune responses, but cannot cause the disease itself. Examples of inactivated vaccines include hepatitis A, influenza (shot only), polio (shot only) and rabies.

Subunit vaccines use only a part of the virus or bacteria is included in the vaccine instead of the the full germ. Because these vaccines contain only the essential antigens and not all the other molecules that make up the germ, they cannot cause illness. The pertussis (whooping cough) component of the DTaP vaccine is an example of a subunit vaccine.

Conjugate vaccines use part of the sugar-like coating of bacteria called polysaccharides. However, because young children don’t make a very good immune response to the sugar coating alone, the coating is linked (conjugated) to a harmless protein. This protein carries the sugar-like coating of the bacteria to certain cells in the immune system to which it would not have access on its own. Then, if the bacteria enters the body, the antibodies will recognize the sugar coating and keep the bacteria from causing disease. Although many conjugate vaccines were developed because of the need to protect infants and young children with immature immune systems, conjugate vaccines are recommended for all ages. Examples of conjugate vaccines include Hib, hepatitis B, HPV, DTaP, shingles, pneumococcal (PCV13) and meningococcal (MenACY).

A mRNA vaccine is a new type of vaccine that protects against infectious diseases. mRNA vaccines teach our cells how to make a protein – or a piece of a protein – that triggers an immune response (antibodies) in our bodies. These antibodies then protect us from future infections.

Our body’s cells break down and get rid of the mRNA soon after it is finished using the instructions on how to make a the protein that triggers our body’s immune response.

COVID-19 vaccines created by Pfizer/BioNTech and Moderna are mRNA vaccines.

Check out the video below from vaccine expert Dr. Paul Offit from the Vaccine Education Center at Children’s Hospital of Philadelphia (CHOP).