How do COVID-19 vaccines work?

A vaccine is medicine used to boost your body’s immune system to prevent a disease. COVID-19 vaccines give you immunity to the virus that causes COVID-19.

Learn about what vaccines do, how they do it, and how the authorized vaccines prevent COVID-19.

What vaccines do we have to protect against COVID-19?

There are currently three vaccines for COVID-19: the Pfizer-BioNTech (Comiranty) vaccine is fully approved for standard use by the U.S. Food and Drug Administration (FDA) for individuals 16 and older, and authorized for emergency use for those aged 5-15, and the Moderna and Johnson & Johnson (Janssen) vaccines are both authorized for emergency use.  

The Pfizer-BioNTech (Comiranty) and Moderna vaccines are both mRNA vaccines and require two doses. The Johnson & Johnson (Janssen) vaccine is an adenoviral vaccine and requires only one dose. 

All of these vaccines are very effective at preventing severe disease and hospitalization. 

What is emergency use authorization (EUA)?

In certain types of emergencies, the Food and Drug Administration (FDA) can issue an emergency use authorization (EUA) to provide more timely access to drugs, diagnostic tests, or other critical medical products that may help during an emergency, like the COVID-19 pandemic.

Learn more about EUAs in this video

How does an mRNA vaccine work?

Watch Pat Winokur, MD, executive dean for the Carver College of Medicine and principal investigator for UI Health Care’s Pfizer-BioNTech COVID-19 vaccine clinical trial, explain what an mRNA vaccine (like Moderna or Pfizer-BioNTech [Comiranty]) is and how it protects your body against the coronavirus:

What are the common side effects of the COVID-19 vaccine?

Early indications are that mild to moderate flu-like side effects might occur, such as arm pain, headache, fatigue, fever, or chills, lasting up to 48 hours. This is your body creating a response to the vaccine and is normal.

If I get the vaccine, does it provide protection to my children?

We anticipate that this will be the case, though we are waiting for the data from studies to prove that the vaccine reduces spread of disease from person to person.

Do you gain some immunity to the virus after the first vaccination, or only after the second vaccination?

It typically takes two weeks after vaccination for the body to build protection (immunity) against the virus that causes COVID-19. That means it is possible a person could still get COVID-19 before or just after vaccination because the vaccine did not have enough time to provide protection. People are considered fully protected two weeks after their second dose of the Pfizer-BioNTech (Comiranty) or Moderna COVID-19 vaccine, or two weeks after the single-dose Johnson & Johnson (Janssen) COVID-19 vaccine. Source: CDC

Learn why getting the second dose of your COVID-19 vaccination matters

I didn't experience any side effects–how do I know the vaccine worked?

Watch Melanie Wellington, MD, PhD, explain how you’re still protected from COVID-19 even if you didn’t experience side effects from the vaccination:

Last reviewed: 
November 2021

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