Merck Manual

Please confirm that you are not located inside the Russian Federation

honeypot link

COVID-19 Vaccine

By

Margot L. Savoy

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

Last full review/revision Jun 2021| Content last modified Jun 2022
CLICK HERE FOR THE PROFESSIONAL VERSION

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 the coronavirus named SARS-CoV-2. Symptoms of COVID-19 vary significantly. Two types of tests can be used to diagnose... read more is the disease caused by infection with the SARS-CoV-2 virus. There are multiple COVID-19 vaccines currently in use worldwide (see COVID-19 Vaccine Tracker). This topic includes only those vaccines currently in use in the United States.

There are two mRNA vaccines approved for use in the United States for COVID-19:

  • BNT162b2 COVID-19 vaccine (mRNA) (made by Pfizer-BioNTech)

  • mRNA-1273 COVID-19 vaccine (mRNA) (made by Moderna)

There is one adenovirus vector vaccine available for use in the United States for COVID-19:

  • Ad26.COV2.S COVID-19 vaccine (adenovirus vector) (made by Janssen/Johnson & Johnson)

In the United States, the BNT162b2 vaccine made by Pfizer-BioNTech (Comirnaty) is approved for use in people 16 years of age and older by the U.S. Food and Drug Administration (FDA). This vaccine also has Emergency Use Authorization (EUA) from the FDA for children 6 months to 15 years of age. The mRNA-1273 vaccine made by Moderna (Spikevax) is approved for use in people 18 years of age and older. This vaccine also has EUA for children 6 months to 17 years of age. The Ad26.COV2.S vaccine made by Janssen/Johnson & Johnson has EUA for people 18 years of age and older who do not have access to or cannot receive other COVID-19 vaccines, or who opt to receive Ad26.COV2.S because they would otherwise not receive any COVID-19 vaccine. (See FDA Limits Use of Janssen COVID-19 Vaccine to Certain Individuals.)

Vaccination remains the most effective strategy at preventing severe illness and death resulting from COVID-19 infection with the recent and currently circulating variants, Delta and Omicron. Hospitalization rates in the United States in the fall of 2021 were 8 to 10 times higher in unvaccinated people than vaccinated people. Unvaccinated people were also 20 times more likely to die of COVID-19 than vaccinated people in that time frame. (For more on vaccine effectiveness, see COVID-19 prevention Prevention COVID-19 is an acute respiratory illness that can be severe and is caused by the coronavirus named SARS-CoV-2. Symptoms of COVID-19 vary significantly. Two types of tests can be used to diagnose... read more .)

In most situations, the mRNA vaccines are preferred over the adenovirus vector vaccine for primary series and booster doses because some research shows a relationship between the adenovirus vector vaccine and a rare and serious side effect called vaccine-induced thrombotic thrombocytopenia Side Effects 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... read more . (See Johnson & Johnson’s Janssen COVID-19 Vaccine Overview and Safety.)

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

Administration of COVID-19 Vaccine

The BNT162b2 COVID-19 vaccine (mRNA) made by Pfizer-BioNTech is given as a primary series of 2 injections given at least 3 to 8 weeks apart in people 5 years of age and older.

The BNT162b2 vaccine is given as a primary series of 3 injections in children 6 months to 4 years of age. The second injection is given at least 3 to 8 weeks after the first one. The third injection is given at least 8 weeks after the second one.

The mRNA-1273 COVID-19 vaccine (mRNA) made by Moderna is given as a primary series of 2 injections given at least 4 weeks apart in people 6 months of age and older.

The Ad26.COV2.S COVID-19 vaccine (adenovirus vector) made by Janssen/Johnson & Johnson has EUA for use in people 18 years of age and older who do not have access to or cannot receive other COVID-19 vaccines, or who opt to receive Ad26.COV2.S because they would otherwise not receive any COVID-19 vaccine. A primary series of this vaccine requires only 1 injection.

Additional primary series and booster dose guidance depends on a person's age and on whether they have a weakened or impaired immune system (immunocompromise).

Primary series guidance for people who are moderately to severely immunocompromised

An additional primary series dose is advised for people with a moderately to severely weakened or impaired immune system. People should talk to their health care practitioner about their medical condition and about whether getting an additional primary series dose is appropriate for them. (See Guidance for COVID-19 vaccination for people who are moderately or severely immunocompromised.) The additional primary series dose applies to the following people:

  • Moderately to severely immunocompromised people 6 months of age and older who completed the primary series of the mRNA-1273 vaccine (made by Moderna) should get an additional primary dose at least 4 weeks after receiving their second shot.

  • Moderately to severely immunocompromised people 5 years of age and older who completed the primary series of the BNT162b2 vaccine (made by Pfizer-BioNTech) should get an additional primary dose at least 4 weeks after receiving their second shot.

  • Moderately to severely immunocompromised people 18 years of age and older who received the Ad26.COV2.S vaccine (made by Janssen/Johnson & Johnson) should get a second shot (additional dose), preferably using an mRNA COVID-19 vaccine, at least 4 weeks after receiving their Ad26.COV2.S shot.

After a primary series is completed, protection against infection has been shown to decrease over time. To maximize protection against infection, severe disease, and death, booster doses are recommended 2 to 5 months after the primary series is completed. People who have received a booster dose when eligible are considered up-to-date on their vaccine series.

COVID-19 booster shot guidance for people who are not immunocompromised

People who are not immunocompromised and who are fully vaccinated against COVID-19 are eligible for a booster shot depending on their age and on the primary series they completed (see COVID-19 Vaccine Booster Shots):

  • A first booster dose of BNT162b2 vaccine is recommended for all BNT162b2 vaccine recipients 5 years of age and older and who completed their primary series 5 or more months ago.

  • A first booster dose of mRNA-1273 vaccine is recommended for all mRNA-1273 vaccine recipients 18 years of age and older and who completed their primary series 5 or more months ago.

  • A first booster dose, preferably of BNT162b2 vaccine or mRNA-1273 vaccine, is recommended for all Ad26.COV2.S vaccine recipients 18 years of age and older and who received their dose 8 or more weeks ago. A second booster dose of BNT162b2 vaccine or mRNA-1273 vaccine may be given to people 18 years of age and older at least 4 months after receiving a first booster dose that followed the Ad26.COV2.S vaccine.

  • A second booster dose of BNT162b2 vaccine or mRNA-1273 vaccine may be given to people 50 years of age and older at least 4 months after receiving a first booster dose of any available COVID-19 vaccine (BNT162b2, mRNA-1273, or Ad26.COV2.S).

Eligible people 18 years of age and older may choose to receive a booster dose of any available COVID-19 vaccine (made by Pfizer-BioNTech, Moderna, or Janssen/Johnson & Johnson), regardless of which vaccine they initially received, but the mRNA vaccines are preferred in most cases because of the risk of vaccine-induced thrombotic thrombocytopenia syndrome with the vector vaccine. Only BNT162b2 vaccine has EUA for booster shots for children 5 to 17 years of age.

COVID-19 booster shot guidance for people who are moderately to severely immunocompromised

  • Moderately to severely immunocompromised people 18 years of age and older who completed the primary series of the BNT162b2 vaccine (produced by Pfizer-BioNTech) or the mRNA-1273 vaccine (produced by Moderna) should get a booster shot of the BNT162b2 or mRNA-1273 vaccine at least 3 months after the third primary dose. A second booster dose may be given at least 4 months after the first booster dose.

  • Moderately to severely immunocompromised children 12 to 17 years of age who completed the primary series of the BNT162b2 vaccine (produced by Pfizer-BioNTech) should get a booster shot at least 3 months after the third primary dose. A second booster dose may be given at least 4 months after the first booster dose.

  • Moderately to severely immunocompromised children 5 to 11 years of age who completed the primary series of the BNT162b2 vaccine should get a booster shot of the BNT162b2 vaccine at least 3 months after the third primary dose.

  • Moderately to severely immunocompromised people 18 years of age and older who received the Ad26.COV2.S vaccine (produced by Janssen/Johnson & Johnson) should get a booster shot at least 8 weeks after the additional primary mRNA COVID-19 vaccine dose. (People who received a primary dose of the Ad26.COV2.S vaccine and already received a first booster dose without having had the mRNA primary dose should get an mRNA vaccine as a third dose at least 2 months after the booster dose.) A second booster dose (fourth shot) with an mRNA COVID-19 vaccine may be given at least 4 months after the prior dose.

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 vaccines made by Pfizer-BioNTech and Moderna, 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 vaccine made by Pfizer-BioNTech or Moderna 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 vaccine made by Janssen/Johnson & Johnson if they have a history of severe allergic reaction to any of its ingredients.

Very rarely, people 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 receiving the vaccine made by Janssen/Johnson & Johnson. In this condition, called vaccine-induced thrombosis with thrombocytopenia syndrome, blood clots develop in unusual and critical places, such as in blood vessels in the brain or abdomen. Symptoms begin about 1 to 2 weeks after vaccination. This problem has occurred in men and women 18 years of age and older but is more likely to occur in women 30 to 49 years of age. Some people die. (See also Johnson & Johnson’s Janssen COVID-19 Vaccine Overview and Safety.)

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 after the second dose of the vaccines made by Pfizer-BioNTech and Moderna, particularly within 7 days of the second dose, suggesting there may be an increased risk of these problems after vaccination. The risk is highest in young males. 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.

More Information

Drugs Mentioned In This Article

Generic Name Select Brand Names
COMIRNATY
NOTE: This is the Consumer Version. DOCTORS: CLICK HERE FOR THE PROFESSIONAL VERSION
CLICK HERE FOR THE PROFESSIONAL VERSION
quiz link

Test your knowledge

Take a Quiz! 
iOS ANDROID
iOS ANDROID
iOS ANDROID
TOP