What is an mRNA vaccine and how does it work?
Most vaccines (or immunizations) work by injecting a weakened form of a disease germ into your body. This is usually done with a shot in the arm or leg. Your body detects these germs and makes antibodies to fight them. Those protective antibodies then stay in your body for a long time. In many cases, they stay for the rest of your life.
A messenger ribonucleic acid (mRNA) vaccine is a different kind of vaccine. It works by giving your cells instructions that tell them to make a special protein called a “spike protein.” Your body’s immune cells respond to this spike protein just like they would to a weakened disease germ from a regular vaccine. They make antibodies to fight it. The spike protein instructions in the COVID-19 mRNA vaccine help your body make antibodies that prevent the virus that causes COVID-19 (called SARS-CoV-2) from entering your cells and causing disease. This keeps you from getting sick.
The instructions from the mRNA vaccine don’t interact with your DNA in any way. In fact, once the instructions are copied, your body quickly breaks down the mRNA and removes it from your system.
Can any of the vaccines cause COVID-19?
No. None of the vaccines are live and they do not replicate so they do not cause disease. The mRNA vaccine doesn’t actually contain the virus that causes COVID-19.
What vaccines are available?
Three vaccines for COVID-19 have been authorized for use in the United States. The U.S. Food and Drug Administration (FDA) gave emergency use authorization (also called an EUA) to the mRNA vaccines developed by Pfizer-BioNTech and Moderna and Novavax.
The Centers for Disease Control and Prevention (CDC) recommends the Pfizer-BioNTech and Moderna vaccines for people 6 months and older and Novavax vaccines for people 18 and older.
Because children and adolescents can get COVID-19 just as easily as adults and can transmit it to others, the CDC now recommends the Pfizer-BioNTech and Moderna COVID-19 vaccine for children 6 months and older. This vaccine is safe and effective, and can reduce severe illness in children.
Why should I get a COVID-19 vaccine?
Adult clinical trials for the vaccines showed that they are all effective at preventing severe COVID-19 disease, hospitalizations, and death due to COVID-19. In children, trials showed the Pfizer-BioNTech vaccine was 90% effective in preventing symptomatic COVID-19 and 100% effective at preventing hospitalization and death.
By getting vaccinated, you are:
- Reducing your risk of serious illness and severe complications, hospitalization, and even death.
- Protecting other vulnerable people in your family and community.
- Helping prevent the health care system from being overwhelmed.
How much does it cost to get the vaccine?
COVID-19 vaccines are available for free to everyone 6 months and older living in the United States, regardless of immigration or insurance status.
How many doses are needed?
In September 2023, the CDC recommended that everyone aged 6 months and older get one dose of the updated 2023-2024 vaccine. Recommendations include:
- Updated 2023-2024 Pfizer-BioNTech and Moderna COVID-19 vaccines (also referred to as mRNA vaccines) for adults and children 5 years and older.
- For children ages 6 months to 4 years with no previous vaccine, the initial vaccination should be either 2 doses of the updated 2023-2024 Moderna or 3 doses of the updated 2023-2024 Pfizer-BioNTech vaccine.
- Children ages 6 months to 4 years with previous mRNA doses need 1 or 2 doses of the updated vaccine, depending on their prior doses.
For people who are moderately or severely immunocompromised:
- Initial vaccination: Should be a 3-dose series of the updated (2023-2024) Moderna or Pfizer-BioNTech vaccine.
- Those who have received previous doses need 1 or 2 doses of the updated vaccine, depending on the number of prior doses.
- Moderately or severely immunocompromised may receive 1 or more additional updated vaccine doses.
What are the side effects of the vaccines?
The vaccines can cause mild symptoms in some people. This happens because the spike protein instructions tell the immune cells to make antibodies. The most common reactions to the vaccine are pain at the injection site, fever, fatigue, headache, and muscle aches. These side effects are very common with other vaccines, including the flu shot. They are a sign that the body is responding the way it should to a vaccine.
In rigorous clinical trials, the vaccines showed no serious safety concerns or severe side effects. However, the CDC and the FDA have continued to monitor for side effects as the vaccine is distributed to the public. There have been a small number of people who had a severe allergic reaction (anaphylaxis) to one of the vaccines, but this is rare, and most people will not have any issues. But you will be asked about any history of reactions to vaccines or other injections just to be safe. If you have concerns or questions about any side effects after getting the vaccine, check with your family doctor.
How long does immunity last?
We still don’t know how long immunity lasts after you get a COVID-19 vaccine. We also don’t know how long immunity lasts after natural infection with the virus that causes COVID-19. We do know that being infected with seasonal coronaviruses (a source for the common cold) does not give long-lasting immunity. It’s why seasonal colds are so common every year.
Do I still need to wear a mask and physically distance if I’m fully vaccinated and have gotten a booster?
The vaccine protects you from getting very sick by keeping the SARS-CoV-2 virus from entering your cells and causing COVID-19. But the vaccine does not keep you from getting the virus and possibly spreading it. Also, it takes a few weeks after the second dose of the vaccine to get the best protection.
Even if you’re vaccinated against COVID-19, when contagious variants are circulating, masks and physical distancing can help. Unless there are local requirements by the local health department or school district, this is a personal choice for your family. But these methods can prevent you from giving the virus to others and protects you by reducing the amount of virus exposure.
Should I get the vaccine if I’ve already had COVID-19?
Yes, as long as you are not currently sick. We know that natural immunity to the virus that causes COVID-19 decreases over time. There’s not enough information from the COVID-19 vaccine studies to show how immunity from the vaccine compares to natural immunity. It’s likely the vaccine will boost your immunity and keep you protected longer.
Who can’t get the vaccine?
A vaccine is currently not available for children under the age of 6 months. People should talk with their family doctor before getting the COVID-19 vaccine. This includes people who have severe reactions to injected medicines, and people who have weakened immune systems and chronic conditions.
As with other vaccines, people who have an allergy to any part of the vaccine should not get the shot. People who have a fever or other symptoms may not be able to get the vaccine until they’re better.
Growing evidence indicates the vaccine is safe for people who are pregnant and breastfeeding. It does not cause fertility problems. The CDC now strongly recommends that all people who are pregnant, recently pregnant, are trying to get pregnant or might become pregnant get vaccinated.
How do I report side effects after getting the vaccine?
If you have side effects (also called adverse events) after getting the COVID-19 vaccine, you should report them to the Vaccine Adverse Event Reporting System (VAERS). Talk with your family doctor if you have questions about what might be considered a side effect related to the vaccine.
The CDC will also be using a new smartphone-based tool called v-safe to send text messages to encourage people to report any side effects. Information on v-safe will be given to everyone who gets the COVID-19 vaccine. You will also get a card that tells you which vaccine and dose you were given and provides other information about the vaccine. You must opt in to get v-safe text messages on your smartphone.
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This article was contributed by familydoctor.org editorial staff.
Copyright © American Academy of Family Physicians
This information provides a general overview and may not apply to everyone. Talk to your family doctor to find out if this information applies to you and to get more information on this subject.
