mRNA Vaccines: How They Work, Benefits, Side Effects and Safety
Updated: 6 days ago
Written and medically reviewed by Dr. Baraa Alnahhal, MD · Last reviewed: October 2026 · How we review
Quick Answer: What is an mRNA vaccine?
An mRNA vaccine is a preventive shot that gives your cells temporary instructions, called messenger RNA, to make one harmless piece of a germ so your immune system learns to recognize it. Today, mRNA vaccines are used against COVID-19 and RSV. Common side effects are arm pain, fatigue and headache; trouble breathing or facial swelling needs emergency care. [1]
What the research says
Two doses gave 95% protection in the original trial. In a randomized trial of 43,548 people aged 16 and older, there were 8 COVID-19 cases in the vaccine group versus 162 in the placebo group, and side effects were mostly short-term, mild-to-moderate arm pain, fatigue and headache. (Polack et al., N Engl J Med, 2020) [2]
Protection against severe illness lasts longer than protection against infection. A review of 123 studies found mRNA booster doses cut severe Omicron infections by 90.5% at 2 weeks and by 77.6% at 26 weeks after vaccination. (Moore et al., Vaccine, 2025) [3]
A first-dose allergic reaction rarely repeats. Among 1,366 people who had an immediate allergic reaction to their first mRNA dose, 99.84% tolerated the second dose, and severe repeat reactions occurred in 0.16%, with no deaths. (Chu et al., JAMA Intern Med, 2022) [4]
Key takeaways
mRNA vaccines contain no virus parts. They carry instructions for making one viral protein. [1]
The vaccine's mRNA breaks down in your body within a few days. [1]
mRNA vaccines in use today include an RSV vaccine (mRESVIA) and COVID-19 vaccines: Comirnaty and Spikevax. [1]
Labs do not need to grow the virus, so mRNA vaccines can be developed quickly. [1]
Most side effects are mild. Trouble breathing or swelling of the face, lips, tongue or throat is an emergency. [1]
What is mRNA and what does it do in the body?
mRNA (messenger RNA) is a molecule that copies instructions from your DNA and carries them to the parts of the cell that build proteins. [1]
Those protein-building parts are called ribosomes. They read the mRNA message and make the proteins your body needs to work. [1]
The mRNA in a vaccine follows the same process. It carries instructions for making one part of a germ, so your immune system can learn what that germ looks like. [1]
Your body does not keep this mRNA. It breaks down within a few days. [1]
Want the bigger picture of how DNA, genes and proteins connect? Read our guide to genes, the blueprints of life.
How does an mRNA vaccine work?
An mRNA vaccine works by teaching your cells to make one harmless piece of a virus, which trains your immune system to recognize the real virus later. [1]
For COVID-19, the mRNA tells your cells to make the spike protein, the part that sits on the surface of the SARS-CoV-2 virus. [1]
Your immune system sees the spike protein as foreign. It responds by making antibodies and other defenses. [1]
If you meet the real virus later, your immune system already knows it and can respond faster. [1]
Step | What happens |
1. The shot | mRNA is injected, usually into the upper arm |
2. Cells read it | Your cells follow the mRNA instructions |
3. Protein is made | Cells make one viral protein (for COVID-19, the spike protein) |
4. Defenses build | Your immune system makes antibodies and other defenses |
5. mRNA clears | The vaccine's mRNA breaks down within a few days |
Source: [1]
Learn more about how your body's defenders work in our guides to antibodies and the human immune system.
How are mRNA vaccines different from traditional vaccines?
mRNA vaccines differ from traditional vaccines because they contain no virus parts; instead, they give your cells instructions to make one viral protein themselves. [1]
Traditional vaccines use a killed or weakened virus, or parts of a virus, to train the immune system. [1]
Both approaches have the same goal: to teach your body to recognize a germ before you get sick from it. [1]
Feature | mRNA vaccine | Traditional vaccine |
What is in the shot | Instructions (mRNA) to make one viral protein | Killed or weakened virus, or parts of a virus |
Contains virus parts? | No | Yes |
Who makes the protein | Your own cells | Made before the shot |
Need to grow the virus in a lab? | No | Usually |
Example | COVID-19 vaccines (Comirnaty, Spikevax) | Novavax COVID-19 vaccine (protein subunit) |
Source: [1]
Which mRNA vaccines are available today?
mRNA vaccines in use today include the RSV vaccine mRESVIA (Moderna) and these COVID-19 vaccines: Comirnaty (Pfizer-BioNTech) and Spikevax (Moderna). [1]
Not every COVID-19 vaccine uses mRNA. The Novavax vaccine is a protein subunit vaccine, not an mRNA vaccine. [1]
Scientists are studying how mRNA technology could be used in vaccines for other infectious diseases and cancer. [1]
For details on the COVID-19 shot itself, see our guide to the COVID-19 vaccine and what to expect.
What happens when you get an mRNA vaccine?
An mRNA vaccine is given as a shot by a healthcare provider, usually in the upper arm, or in the leg for young children. [1]
After the shot, you may be asked to wait at least 15 minutes. This lets the care team check for an immediate allergic reaction. [1]
What are the benefits of mRNA vaccines?
The main benefits of mRNA vaccines are faster development and strong protection against serious COVID-19. [1]
Faster to make. Labs don't need to grow copies of the virus, so new vaccines can be developed quickly. [1]
Lower risk of infection and serious illness. Current COVID-19 vaccines can reduce your risk of getting infected, becoming seriously ill and being hospitalized. [1]
Lower risk of long COVID. Vaccination can also reduce the risk of long COVID. [1]
In the original large trial, two doses were 95% effective at preventing COVID-19. [2] Protection against severe illness also held up better over time than protection against infection. [3]
Read more about the disease itself in our COVID-19 guide.
What are the side effects and risks of mRNA vaccines?
The most common side effects of mRNA vaccines are pain or swelling at the injection site, fatigue, headache, muscle aches and fever. [1]
In the original trial, these reactions were mostly short-term and mild to moderate. [2]
Some people can have an allergic reaction to an ingredient in the vaccine. Signs include rash, itching, hives and swelling. [1]
A severe allergic reaction (anaphylaxis) can cause swelling of the face, lips, tongue or throat. This is an emergency. Rarely, mRNA COVID-19 vaccines cause heart inflammation (myocarditis or pericarditis), most often in young males within a week of a dose; get care right away for chest pain, shortness of breath or a fast, fluttering or pounding heartbeat. [1]
Type | Signs | What to do |
Common side effects | Arm pain or swelling, fatigue, headache, muscle aches, fever | Talk to your provider if you're worried [1] |
Allergic reaction | Rash, itching, hives | Contact your healthcare provider [1] |
Severe allergic reaction | Trouble breathing, severe swelling, swelling of the face, lips, tongue or throat | Go to the emergency room right away [1] |
When should you call a doctor or get emergency care after an mRNA vaccine?
Go to an emergency room right away if you have signs of a severe allergic reaction after an mRNA vaccine, such as trouble breathing or severe swelling. [1]
Talk to your healthcare provider if you have concerns about mRNA vaccines or which COVID-19 vaccine is right for you. [1]
If you had an immediate allergic reaction to your first dose, ask your provider about next steps. Research shows most people in this situation safely received a second dose in a supervised setting. [4]
Learn to spot the warning signs in our anaphylaxis guide.
How were mRNA vaccines developed?
mRNA vaccines were developed over about 50 years of research, starting in the 1970s. [1]
Year | Milestone |
1970s | Research into mRNA technology begins |
1990s | Scientists begin applying mRNA to vaccines |
2018 | The FDA approves patisiran (Onpattro), a drug for a rare nerve disease that delivers RNA using lipid nanoparticles |
December 2020 | The FDA authorizes the first COVID-19 vaccines for emergency use in the U.S. |
Source: [1]
The lipid nanoparticle delivery method used for patisiran paved the way for the mRNA COVID-19 vaccines. [1]
mRNA vaccines at a glance
Question | Answer |
What is it? | A preventive shot that teaches your cells to make one harmless viral protein |
What's inside? | mRNA instructions, no virus parts |
How long does the mRNA stay? | It breaks down within a few days |
Which vaccines use it? | COVID-19 vaccines (Comirnaty, Spikevax) and an RSV vaccine (mRESVIA) |
How is it given? | A shot in the upper arm (or leg for young children) |
Common side effects | Arm pain or swelling, fatigue, headache, muscle aches, fever |
Emergency signs | Trouble breathing, swelling of the face, lips, tongue or throat |
Source: [1]
Conclusion
mRNA vaccines give your cells short-lived instructions to make one harmless piece of a virus. Your immune system learns from it, and the mRNA breaks down within days. [1]
Today these vaccines protect against COVID-19, lowering the risk of infection, serious illness, hospitalization and long COVID. [1] Most side effects are mild, but trouble breathing or swelling of the face or throat needs emergency care. [1]
If you have questions about which vaccine is right for you, bring this guide to your next appointment and talk it through with your healthcare provider.
Keep learning with Rinnit. Explore more plain-language, physician-reviewed guides in our health library, and share this article with someone who has questions about mRNA vaccines.
FAQ
What is an mRNA vaccine?
An mRNA vaccine is a preventive shot that gives your cells instructions to make one harmless piece of a germ. Your immune system learns to recognize that piece and can fight the real germ faster later. [1]
Do mRNA vaccines contain the virus?
mRNA vaccines do not contain any virus parts. They contain instructions that tell your cells to make one viral protein, such as the COVID-19 spike protein. [1]
How long does mRNA from a vaccine stay in the body?
mRNA from a vaccine breaks down in your body within a few days. [1]
Are all COVID-19 vaccines mRNA vaccines?
Not all COVID-19 vaccines are mRNA vaccines. Comirnaty and Spikevax use mRNA, while the Novavax vaccine is a protein subunit vaccine. [1]
What are the common side effects of mRNA vaccines?
The common side effects of mRNA vaccines are pain or swelling at the injection site, fatigue, headache, muscle aches and fever. In the original trial, most were short-term and mild to moderate. [1][2]
Why do I have to wait after an mRNA vaccine?
Waiting at least 15 minutes after an mRNA vaccine lets your care team check for an immediate allergic reaction. [1]
When should I seek emergency care after an mRNA vaccine?
Seek emergency care right away if you have signs of a severe allergic reaction after an mRNA vaccine, such as trouble breathing or swelling of the face, lips, tongue or throat. [1]
Are mRNA vaccines being developed for other diseases?
mRNA vaccines are being studied for other infectious diseases and for cancer. Right now, mRNA vaccines are in use for COVID-19 and RSV. [1]
Can an mRNA vaccine change your DNA?
No. The mRNA never enters the part of the cell where DNA is kept, and your body breaks it down within days.
Can you get COVID-19 from an mRNA vaccine?
No. mRNA vaccines contain no live virus, so they cannot give you COVID-19. Feeling tired or achy for a day or two is a normal sign that your immune system is responding.
Related Reading
Vaccines: How They Work, Types, Safety and Side Effects explains how vaccines work and why they matter.
References
Cleveland Clinic. mRNA Vaccines: What They Are & How They Work. Cleveland Clinic. Last updated October 25, 2024. https://my.clevelandclinic.org/health/treatments/21898-mrna-vaccines
Polack FP, Thomas SJ, Kitchin N, et al. Safety and Efficacy of the BNT162b2 mRNA Covid-19 Vaccine. N Engl J Med. 2020;383(27):2603-2615. PMID 33301246. https://doi.org/10.1056/NEJMoa2034577
Moore M, Anderson L, Schiffer JT, et al. Durability of COVID-19 vaccine and infection induced immunity: A systematic review and meta-regression analysis. Vaccine. 2025;54:126966. PMID 40048931. https://doi.org/10.1016/j.vaccine.2025.126966
Chu DK, Abrams EM, Golden DBK, et al. Risk of Second Allergic Reaction to SARS-CoV-2 Vaccines: A Systematic Review and Meta-analysis. JAMA Intern Med. 2022;182(4):376-385. PMID 35188528. https://doi.org/10.1001/jamainternmed.2021.8515
Medical disclaimer
This article is for general education only and is not a substitute for professional medical advice, diagnosis or treatment. Always talk to a qualified healthcare provider about your health and before making decisions about vaccination. If you think you are having a medical emergency, call your local emergency number right away.
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