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COVID-19 Vaccine


Margot L. Savoy

, MD, MPH, Lewis Katz School of Medicine at Temple University

Last full review/revision Jun 2021| Content last modified Jun 2021

Coronavirus disease 2019 (COVID-19) vaccines provide protection against COVID-19. COVID-19 is the disease caused by infection with the SARS-CoV-2 virus. There are multiple COVID-19 vaccines currently in use worldwide. This topic includes only those vaccines currently in use in the United States. In the United States, there are no vaccines that have been approved by the U.S. Food and Drug Administration (FDA). However, there are three vaccines that have received Emergency Use Authorization (EUA) from the FDA:

  • Pfizer-BioNTech
  • Moderna
  • Janssen (also known as the Johnson & Johnson vaccine)

For more information, see the EUA fact sheets for the Pfizer-BioNTech, Moderna, and Janssen (Johnson & Johnson) vaccines.

All 3 vaccines completely prevented serious complications of COVID-19 including hospitalizations and deaths in clinical trials. In a clinical trial, the Pfizer-BioNTech vaccine prevented COVID-19 disease in 95% of people following 2 doses given 3 weeks apart. In a separate trial, the Moderna vaccine prevented COVID-19 disease in 94.1% of people. The Janssen vaccine prevented COVID-19 disease in about 67% of people overall and prevented severe/critical COVID-19 disease in 85% after one dose. It is important to note that these trials cannot be compared directly because they were done on different groups of people at different points in the pandemic. The duration of the protection from the vaccines is currently not known. People with a weakened immune system, including those taking immunosuppressant drugs, may have a diminished response to the vaccine. Although these vaccines decrease the likelihood and severity of infection, it is not currently known how well vaccines prevent the spread of the virus that causes COVID-19. Thus, people who have been vaccinated should still follow the same general prevention measures recommended for unvaccinated people in the same region, including mask wearing, social distancing, and frequent hand washing.

(See also Overview of Immunization.)

Administration of COVID-19 Vaccine

The Pfizer-BioNTech vaccine received EUA on December 11, 2020, for use in people 12 years of age and older and is given as a series of 2 injections given 3 weeks apart.

The Moderna vaccine received EUA on December 18, 2020, for use in people 18 years of age and older and also requires 2 injections but given 4 weeks apart.

The Janssen vaccine (Johnson & Johnson vaccine) received EUA on February 27, 2021, for use in people 18 years of age and older and requires only a single injection.

The Pfizer-BioNTech and Moderna COVID-19 vaccines are not interchangeable. People must receive the vaccine from the same manufacturer for both doses.

Side Effects of COVID-19 Vaccine

The three COVID-19 vaccines have similar side effects:

  • Pain, swelling, and redness at the injection site
  • Tiredness
  • Headache
  • Muscle and joint pains
  • Fever and chills
  • Nausea
  • Feeling unwell
  • Swollen lymph nodes

Side effects typically last several days. For the Pfizer-BioNTech and Moderna COVID-19 vaccines, more people have side effects after the second dose than after the first dose.

There is a remote chance of a severe allergic reaction. This usually occurs within a few minutes to 1 hour after getting a dose of the vaccine and requires emergency treatment (call for emergency medical care [911 in the United States] or go to the nearest hospital). People who have had severe allergic reactions to other vaccines or injectable drugs should discuss the risk of an allergic reaction with their doctor and be observed after receiving the vaccine. Signs of a severe allergic reaction include

  • Difficulty breathing
  • Swelling of face and throat
  • A fast heartbeat
  • A bad rash all over body
  • Dizziness and weakness

People should not get the Pfizer-BioNTech or Moderna vaccine if they have had a severe allergic reaction to a previous dose of the vaccine or to any component of the vaccine (including polyethylene glycol [PEG]). People should not get the Janssen vaccine if they have a history of severe allergic reaction to any of its ingredients.

Very rarely, people (mostly women) develop an unusual problem with excessive blood clotting (thrombosis) and low platelet levels (thrombocytopenia) after vaccination. In this condition, called vaccine-induced thrombotic thrombocytopenia, blood clots develop in unusual and critical places, such as in blood vessels in the brain or abdomen.

The Centers for Disease Control and Prevention (CDC) states women who are due for a mammogram and who have recently received COVID-19 vaccination ask their doctor how long they should wait after vaccination to get their mammogram because temporary reactions to the vaccine might cause a false reading on the mammogram. Some experts recommend getting the mammogram before the vaccine or waiting 4 to 6 weeks after getting the vaccine.

More Information

The following are English-language resources that may be useful. Please note that THE MANUAL is not responsible for the content of these resources.

  • U.S. Food and Drug Administration (FDA): Fact sheet for recipients and caregivers: Emergency Use Authorization (EUA) of the Pfizer-BioNTech COVID-19 Vaccine
  • FDA: Fact sheet for recipients and caregivers: Emergency Use Authorization (EUA) of the Moderna COVID-19 Vaccine
  • FDA: Fact sheet for recipients and caregivers: Emergency Use Authorization (EUA) of the Janssen COVID-19 Vaccine
  • Centers for Disease Control and Prevention (CDC): Vaccines for COVID-19
  • CDC: COVID-19 vaccination and other medical procedures: Mammograms: Guidance regarding routine mammography before or after COVID-19 vaccination

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