Understanding How COVID-19 Vaccines Work


What You Need to Know

  • COVID-19 vaccines are safe and effective.

  • You may have side effects after vaccination, but these are normal.

  • It typically takes two weeks after you are fully vaccinated for the body to build protection (immunity) against the virus that causes COVID-19.

  • If you are not vaccinated, find a vaccine. Keep taking all precautions until you are fully vaccinated.

  • If you are fully vaccinated, you can resume activities that you did prior to the pandemic. Learn more about what you can do when you have been fully vaccinated.

The Immune System—the Body’s Defense Against Infection

To understand how COVID-19 vaccines work, it helps to first look at how our bodies fight illness. When germs, such as the virus that causes COVID-19, invade our bodies, they attack and multiply. This invasion, called an infection, is what causes illness. Our immune system uses several tools to fight infection. Blood contains red cells, which carry oxygen to tissues and organs, and white or immune cells, which fight infection. Different types of white blood cells fight infection in different ways:

  • Macrophages are white blood cells that swallow up and digest germs and 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 produce antibodies that attack the pieces of the virus left behind by the macrophages.

  • T-lymphocytes are another type of defensive white blood cell. They attack cells in the body that have already been infected.

The first time a person is infected with the virus that causes COVID-19, it can take several days or weeks for their body to make and use all the germ-fighting tools needed to get over the infection. After the infection, the person’s immune system remembers what it learned about how to protect the body against that disease.


The body keeps a few T-lymphocytes, called “memory cells,” that go into action quickly if the body encounters the same virus again. When the familiar antigens are detected, B-lymphocytes produce antibodies to attack them. Experts are still learning how long these memory cells protect a person against the virus that causes COVID-19.


What We Are Still Learning

  • How well the vaccines protect people with weakened immune systems, including people who take medicines that suppress the immune system

  • How long COVID-19 vaccines protect people

  • How many people have to be vaccinated against COVID-19 before the population can be considered protected (population immunity)

  • How effective the vaccines are against new variants of the virus that causes COVID-19


Find COVID-19 Vaccine Near You

Find a COVID-19 Vaccine: Search vaccines.gov, text your ZIP code to 438829, or call 1-800-232-0233 to find locations near you in the U.S.

There are several ways you can look for vaccination providers near you in the United States.

  • Visit Vaccines.gov to find vaccination providers near you. In some states, information may be limited while more U. S. vaccination providers and pharmacies are being added. Learn more about COVID-19 Vaccination Locations on Vaccines.gov.

  • Text your ZIP code to 438829 or call 1-800-232-0233 to find vaccine locations near you in the United States.

  • Check your local pharmacy’s website to see if vaccination appointments are available. Find out which pharmacies are participating in the Federal Retail Pharmacy Program.

  • Contact your state health department to find additional vaccination locations in the area.

  • Check your local news outlets. They may have information on how to get a vaccination appointment.

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