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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 Nov 2021

Coronavirus disease 2019 (COVID-19) vaccines provide protection against COVID-19. COVID-19 COVID-19 COVID-19 is an acute respiratory illness that can be severe and is caused by a newly identified coronavirus officially named SARS-CoV-2. COVID-19 was first reported in late 2019 in Wuhan, China... read more 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, the Pfizer COVID-19 vaccine (mRNA) has been approved for use in people 16 years of age and older by the U.S. Food and Drug Administration (FDA). The Pfizer vaccine also received Emergency Use Authorization (EUA) from the FDA for children 5 to 15 years of age. Two other vaccines, Moderna COVID-19 vaccine (mRNA) and Johnson & Johnson COVID-19 vaccine (adenovirus vector), received EUA for people 18 years of age and older.

For more information, see the FDA news release for the Pfizer-BioNTech vaccine approval and see the EUA fact sheets for the Pfizer, Moderna, and 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 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 Johnson & Johnson 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.

Administration of COVID-19 Vaccine

The Pfizer-BioNTech COVID-19 vaccine (mRNA) has FDA approval for use in people 16 years of age and older and is given as a series of 2 injections given 3 weeks apart.

The Pfizer-BioNTech COVID-19 vaccine (mRNA) received EUA for use in people 5 to 15 years of age and is given as a series of 2 injections given 3 weeks apart.

The Moderna COVID-19 vaccine (mRNA) received EUA for use in people 18 years of age and older and also requires 2 injections but given 4 weeks apart.

The Johnson & Johnson COVID-19 vaccine (adenovirus vector) received EUA for use in people 18 years of age and older and requires only a single injection.

An additional dose of the Pfizer or Moderna vaccine received EUA and is recommended for people with a moderately to severely weakened or impaired immune system, such as those who have undergone solid organ transplantation or have an equivalent level of immunocompromise. This additional dose should be given at least 28 days after a second dose of the Pfizer or Moderna vaccine.

Third doses (booster doses) are available for all Pfizer and Moderna vaccine recipients who are 18 years of age and older and who completed their initial 2-dose series 6 or more months ago.

Second doses (booster doses) are recommended for Johnson & Johnson vaccine recipients who are 18 years of age or older and who received their first dose of the vaccine 2 or more months ago.

More detailed explanations about boosters can be found on the CDC web site. Eligible people may choose to receive a booster dose of any available COVID-19 vaccine (Pfizer, Moderna, or Johnson & Johnson), regardless of which vaccine they initially received.

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 Overview of Thrombocytopenia Thrombocytopenia is a low number of platelets (thrombocytes) in the blood, which increases the risk of bleeding. Thrombocytopenia occurs when the bone marrow makes too few platelets or when... read more Overview of 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 heart problems myocarditis Myocarditis Myocarditis is inflammation of the muscle tissue of the heart (myocardium) that causes tissue death. Myocarditis may be caused by many disorders, including infection, toxins and drugs that affect... read more and pericarditis Acute Pericarditis Acute pericarditis is inflammation of the pericardium (the flexible two-layered sac that envelops the heart) that begins suddenly, is often painful, and causes fluid and blood components such... read more Acute Pericarditis have been reported following the second dose of the Pfizer and Moderna vaccines, suggesting there may be an increased risk of these problems after vaccination. The risk is highest in men 18 to 24 years of age. Vaccine recipients should seek medical attention right away if they have chest pain, shortness of breath, or feelings of having a fast-beating, fluttering, or pounding heart after vaccination.

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.

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