Understanding the mRNA COVID-19 Vaccine

Vaccines for COVID-19 are available. What do you need to know about getting an mRNA COVID-19 vaccine? What can you expect after you get it? Read on to learn more.

mRNA COVID-19 vaccine fast facts

  • Two mRNA vaccines are available: Pfizer and Moderna.

  • Pfizer and Moderna vaccines are available for people as young as 6 months old. They are given in several doses a few weeks apart depending on age and overall health. These first shots are called the primary series. People with a weak immune system have different advice. Talk with your healthcare provider to find out which vaccine is best for you and your family.

  • The vaccine is given as a shot in a muscle.

  • The mRNA vaccine does not use live, dead, or weak COVID virus.

  • The vaccine will not give you COVID-19.

  • Side effects of the vaccine mean your immune system is working, not that you have the virus.

  • Boosters can help with protection against COVID-19 that may have decreased over time after getting the primary series. People age 5 or older can get a COVID-19 booster shot several months after completing their primary series. Talk with you or your child's provider to learn more.

What does the COVID-19 vaccine do?

The COVID-19 vaccine has been shown to work well to prevent symptomatic and severe COVID-19 illness. Getting a COVID-19 vaccine can help protect you and your family from getting ill from the virus. If you get the virus after you get the vaccine, it may help your symptoms be milder. It also lowers your chance of needing a hospital stay because of COVID-19. The COVID-19 vaccine may also help protect people around you from getting the infection.

COVID-19 vaccines may also lead to more widespread changes. The more people who get the COVID-19 vaccine, the less likely the virus will be able to spread in the community. This is called herd immunity or community immunity.

Should I get a COVID-19 vaccine?

CDC advises COVID-19 vaccines and boosters for everyone 6 months and older . This includes those who are pregnant or breastfeeding. The most important thing to do is talk with your healthcare provider about your risks and which vaccine is best for you and your family.

People who have had COVID-19 may still benefit from the vaccine. Researchers don’t yet exactly know how long natural immunity lasts after you have COVID-19. If you recently had COVID-19, you still need stay up to date with your vaccines. But you may be able to delay your next vaccine dose (whether a primary dose or booster) by 3 months from when your symptoms started or when you first had a positive COVID-19 test. Talk with your healthcare provider

Tell your healthcare provider if you have ever had a severe allergic reaction to food or medicine. Talk with them about your risks if you carry an epinephrine autoinjector. This may affect your provider’s advice to you about the vaccine.

How does an mRNA COVID-19 vaccine work?

The COVID-19 mRNA vaccines are different from traditional vaccines. They’re not made with live, dead, or weak virus. Instead, they're made with messenger ribonucleic acid (mRNA). This is a type of molecule that gives instructions for how to make different kinds of proteins. mRNA molecules are a natural part of our cells and how our bodies work.

The mRNA in the vaccines tells your cells how to make a harmless piece of a protein called a spike protein. This protein is found on the outside of the SARS-CoV-2 virus that causes COVID-19. Your immune system sees this spike protein as a threat, and creates antibodies and other defenses against it.

This will help your body's immune system recognize and fight the real virus if it ever shows up. It’s kind of like recognizing someone by the hat they wear. Your body is then prepared to spot COVID-19 and fight it off before it grows in your body’s cells.

How were the COVID-19 vaccines approved for safety?

The COVID-19 mRNA vaccines have passed many tests in labs and in tens of thousands of people and meet strict standards from the FDA.

The vaccines were tested first in animals. They were then tested in a series of clinical trials that included tens of thousands of people. All of the data from these tests was collected and submitted to the FDA and other scientific groups. These committees of scientists and public health experts carefully look at the data to see if a vaccine is safe and effective. If the vaccine meets the FDA's strict standards of safety and quality, the agency tells the vaccine company they can make the vaccine for emergency use.

How much does the vaccine cost?

The U.S. government is providing the vaccine free to U.S. residents. But the site where you get your vaccine may bill your health insurer for giving you the vaccine. Talk with your health insurer, local pharmacy, employer, or healthcare provider to find out more about a possible fee. You can’t be denied a vaccine if you don’t have health insurance or can’t pay the fee yourself.

Getting the mRNA COVID-19 vaccine

The vaccine is given as a shot in a muscle. You will need to have at least 2 doses, spaced several weeks apart. Booster doses are advised several months later to offer longer protection. You need all doses to get the best COVID-19 protection from the vaccine.

Booster advice varies by vaccine, age, health, and COVID-19 variants. People ages 65 and older and some people with a very weak immune system are advised to get an updated (bivalent) mRNA booster. Talk with your provider about your risk and how this applies to you.

Follow instructions from the healthcare staff. Tell the staff if you have ever had a severe allergic reaction to food or medicine or carry an epinephrine autoinjector. Tell them if you feel any reaction after you have the shot. You may be asked to stay for some time after getting the vaccine so you can be monitored.

Side effects: What to expect

The vaccine will have side effects for some people. A vaccine activates a person’s immune system. It causes the immune system to create antibodies to fight off a specific virus or bacteria. When your immune system goes into action, you may feel your immune system kick into gear as though it’s fighting an illness. This does not mean you are infected with an illness. It means that your immune system is working.

People getting the mRNA COVID-19 vaccines often have soreness where the shot was given, tiredness, headaches, muscle and joint aches, chills, and fever for a day or two. Fewer people have redness and swelling at the injection site. These are all signs that your immune system is working on its defense. You can get these kinds of effects after many kinds of vaccines. But these symptoms should last a short time. In comparison, COVID-19 symptoms can be severe and last much longer, and cause complications, long-term illness, and death. The FDA approval process makes sure that the discomfort and risks of a vaccine outweigh the risks and complications of the illness it helps prevent.

Allergic reactions

In general, the COVID-19 vaccines are very safe. They have been tested on thousands of people and given to more than a billion people. Non-severe allergic reactions have happened in a few people up to 4 hours after getting the vaccine. The vaccine clinic may ask you to stay on-site for a period of time after you get the vaccine. This is to make sure you don't have an immediate reaction.

Talk with your healthcare provider before you get a COVID-19 vaccine. Tell them if you have ever had an immediate reaction to any vaccine, even if the reaction was not severe. Your provider will help you weigh the risks and benefits of the COVID-19 vaccine for you.

If you have an allergy to any ingredient in the mRNA COVID-19 vaccine, the CDC advises that you not get the vaccine. If you have had 1 dose of COVID-19 vaccine and you have an immediate allergic reaction, the CDC advises that you not get the second dose.

The CDC has a smartphone app called V-Safe to help you report side effects. The app will also send you reminders if you need a second vaccine dose. To access this app, see "To learn more" below.

Severe symptoms

If you get a COVID-19 vaccine and think you may be having a severe allergic reaction after leaving the vaccine clinic, call 911. Severe symptoms include:

  • Trouble breathing

  • Wheezing

  • Trouble swallowing or feeling like your throat is closing

  • Cool, moist, pale, or blue-tinted skin

  • Hoarse voice or trouble speaking

  • Chest pain

  • Fainting

  • Swelling in the eyes, mouth, face, or tongue

  • Seizure

  • Feeling very drowsy or having trouble awakening

  • Fast heart rate

  • Nausea, vomiting, diarrhea, stomach cramps, or abdominal pain

After you get the COVID-19 vaccine

When you get both doses and boosters of the vaccine:

  • It’s still possible to get COVID-19. Like most vaccines, the COVID-19 vaccines are not 100% effective at preventing the disease. You should still take care to prevent contact with sick people and follow local advice about staying safe.

  • Follow your local, state, and national instructions about wearing a mask and social distancing. Check the most current CDC guidelines .

Date last modified: 4/24/2023

Online Medical Reviewer: Barry Zingman MD
Online Medical Reviewer: L Renee Watson MSN RN
Online Medical Reviewer: Rita Sather RN
Date Last Reviewed: 9/1/2022
© 2000-2024 The StayWell Company, LLC. All rights reserved. This information is not intended as a substitute for professional medical care. Always follow your healthcare professional's instructions.