COVID vaccines train your immune system to fight off COVID-19. Most work by giving your body a set of instructions (mRNA) to make a harmless piece of the virus for your immune system to recognize. Adults and kids 6 and older are fully vaccinated with one bivalent shot. You might have some side effects like a sore arm, muscle aches or fatigue.
COVID vaccines are preventive treatments that train your body to recognize and quickly fight a COVID-19 infection. This means if you’re exposed to COVID, you might not get sick or you’ll get less severely sick than you would have without being vaccinated.
The U.S. Food and Drug Administration (FDA) and World Health Organization (WHO) authorized the first COVID vaccines for emergency use in 2020. They were effective against the original strain of COVID. Since then, SARS-CoV-2, the virus that causes COVID, has changed (mutated). The Omicron variant is different enough from the original strain that the first vaccines aren’t as effective against it. Some manufacturers have created updated vaccines (bivalent vaccines) that train your immune system to recognize both the original and the Omicron variant.
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For adults and children 6 years and older with a healthy immune system, one dose of a bivalent (updated) Moderna® or Pfizer-BioNTech® vaccine is considered fully vaccinated. This is true whether you got an original vaccine series or not.
Adults 65 and older can get one additional dose of the updated vaccine. You should wait at least four months after your first updated vaccine to get the booster.
Kids 6 months to 4 years old are fully vaccinated if they’ve received three Pfizer-BioNTech COVID-19 shots or two Moderna shots, as long as at least one is a bivalent dose.
Five-year-olds are up to date if they got two Moderna shots (as long as at least one is a bivalent dose) or one bivalent Pfizer-BioNTech shot.
If you have a weakened (compromised) immune system, you’re at a higher risk of getting severely ill with COVID. Your immune response to vaccines may not be as strong as it should be, meaning you might need more doses to be protected.
For kids and adults ages 6 months and older who have moderately or severely compromised immune systems, the Centers for Disease Control and Prevention (CDC) recommends:
COVID vaccines, like all vaccines, work by training your immune system to fight off harmful germs (pathogens) that attempt to invade your body. But what does that mean? First, we have to understand how your immune system fights off viruses, bacteria and other pathogens.
Each pathogen has a unique part that your body recognizes as an invader, called an antigen. It’s like a distinctive birthmark or tattoo you look for to identify someone. The first time an invader like a virus or bacteria enters your body, your immune system needs to look for the right tools (specific B-cells) to recognize the antigen and destroy the pathogen it belongs to. When your immune cells find the right tools, they make a lot of them to find and get rid of the infection. But this process can take some time.
You also have special cells that remember the pathogen (memory B-cells). Like taking a photo and putting it on a “wanted” poster, your immune cells can then patrol your body, looking for familiar pathogens. If they encounter one, they can destroy it much more quickly than they did the first time it infected you — often before it makes you sick at all. This is called adaptive immunity.
COVID vaccines aim to train your body to know what SARS-CoV-2 looks like without actually getting infected. Then your immune system can build up its tools and surveillance team, so when it sees the virus, it can fight it off quickly. For some people, this means they don’t get sick at all if they’re exposed to COVID. Other times, it makes their symptoms less severe and allows them to recover more quickly.
Four different manufacturers make COVID vaccines in the U.S. They fall into three categories based on the method they use to get your body to recognize the spike protein of SARS-CoV-2:
COVID’s antigen is a protein that sticks out all around the outside of the virus (the spike protein). COVID vaccines use various techniques to get your body to recognize the spike protein.
mRNA vaccines don’t use a part of the virus to train your immune system. Instead, they give your body instructions that it uses to manufacture harmless proteins that look like parts of the virus (in the case of COVID, the spike protein). Moderna and Pfizer-BioNTech vaccines are mRNA vaccines. Your body uses the mRNA instructions to make the spike protein for your immune system to recognize.
Protein subunit vaccines use a part of the virus to get an immune system response. The Novavax vaccine delivers the spike protein to your cells so they can recognize and be prepared to destroy it if they see it again. Since it’s just part of the virus and not the actual virus itself, it can’t make more copies of itself or hurt you.
Viral vector vaccines use a different, harmless virus to carry a small piece of the genetic code (DNA) for a part of the virus you want to get immunity to. The J&J/Janssen vaccine uses an adenovirus (which typically causes a mild, common cold) to deliver the DNA for COVID’s spike protein to your cells. Your body then reads the DNA and makes instructions for creating the spike protein for your body to recognize. The adenovirus has been modified so it can’t make you sick.
SARS-CoV-2 mutates frequently, making it difficult to create a vaccine that’s effective against all the virus’s different looks. Its identifying feature — the spike protein — now looks different enough from the original “wanted” poster that it can slip past your immune system. So vaccines have been updated with two “wanted” posters: one for the original and one for the Omicron variant. These updated vaccines are called bivalent (the original vaccines are monovalent).
Before getting your COVID shot, you should:
Healthcare providers give all COVID vaccines as injections (shots). In adults and children over 5, a provider gives you the injection into the muscle of your upper arm. In children under 5, the injection is in their thigh (though 3- and 4-year-olds sometimes get it in their arm).
A provider will clean the area with an alcohol swab and inject the vaccine with a needle. They’ll put a bandage over it. Sometimes, they’ll put a small, round bandage (a pre-injection bandage, or an Inject-Safe™ barrier bandage) on first and inject the needle into your skin through the bandage.
Your provider may ask you to wait at least 15 minutes before leaving to make sure you don’t have an immediate allergic reaction.
Studies suggest COVID vaccines are most effective in the first few months following your shot. That’s why, when health experts recommend boosters or updated doses, they’re usually given three to four months after your last COVID shot.
Studies suggest that people who are vaccinated against COVID-19 are less likely than those who aren’t vaccinated to:
Like every medicine, COVID vaccines go through a series of tests to learn whether they’re safe and effective (clinical trials). Thousands of volunteers receive a vaccine before it’s approved for the public. Vaccine manufactures didn’t skip any of these tests before the FDA and other public health organizations around the world approved their COVID vaccines.
A medication that uses the same RNA technology had been tested and approved before COVID vaccines were developed. This made COVID vaccine development faster than vaccines before it.
No matter which type of shot you get, your body breaks down the ingredients or they’re destroyed by your immune system within a few days. This means vaccines can’t cause long-lasting health effects.
All that being said, any vaccine or medication has a risk of side effects and allergic reactions, which can sometimes be severe. Serious health conditions as a result of COVID vaccination are very rare.
The most common risk of getting a COVID vaccine is experiencing unpleasant but harmless side effects. Side effects of the COVID vaccine include:
Side effects like muscle aches, tiredness and fever usually last a day or two. Your arm might hurt for several days.
Serious complications of COVID vaccines are rare. They include:
Some people have an allergic reaction to the ingredients in vaccines. About 1 in 200,000 people who get a COVID shot has anaphylaxis. Anaphylaxis is a life-threatening reaction that causes swelling and extremely low blood pressure. The risk of severe allergic reactions is the reason your provider asks you to wait for 15 minutes after getting your shot — so you can get medical attention right away if you experience an unexpected reaction.
Symptoms of an allergic reaction include:
Guillain-Barré syndrome (GBS) is an autoimmune disorder where your immune system attacks your nerves, damaging them. It can cause numbness, tingling and muscle weakness. In severe cases, it can lead to paralysis.
GBS most often affects men and people assigned male at birth (AMAB) who are 50 years old or older. The J&J/Janssen vaccine has a higher risk of GBS than mRNA vaccines.
Thrombosis with thrombocytopenia syndrome (TTS) is an extremely rare complication of J&J/Janssen and AstraZeneca® vaccines (AstraZeneca vaccines aren’t available and were never used in the U.S.). It activates your platelets, part of your blood that helps it clot.
About 1 in 250,000 people had TTS after getting a COVID vaccine. Nine people died. Moderna and Pfizer-BioNTech vaccines aren’t associated with TTS.
Symptoms of TTS include:
Some people have had an inflammation of their heart muscle (myocarditis) or the outer lining of their heart (pericarditis) after getting an mRNA vaccine. Though rare, it’s most common in men and people AMAB between the ages of 18 and 29 after getting their second shot.
Symptoms of pericarditis and myocarditis include:
You shouldn’t get a COVID vaccine if you:
If you get your shot while you’re sick with COVID, you could end up making yourself feel sicker. You also risk getting other people sick with COVID by going out to get vaccinated.
It’s not necessary to get a COVID vaccine if you’ve recently been sick — it won’t help you get better faster, and you’ll get natural immunity from your body fighting it off. If you’re not up to date on your vaccines, you could consider waiting three months after your symptoms started or after you tested positive to get vaccinated.
Go to an emergency room (ER) immediately if you have signs of a severe allergic reaction, including trouble breathing, severe hives or swelling of your face, lips, tongue or throat.
Talk to your healthcare provider if you:
The FDA authorized the first COVID vaccines for emergency use in the U.S. in December of 2020. It might seem like they came out fast. But decades of research went into developing the technology that now makes vaccines faster and easier to develop.
Research into mRNA technology dates back to the 1970s. Scientists first started applying it to vaccine development in the 1990s. It took over 20 years of research to learn how to get our immune systems to recognize the mRNA without destroying it too quickly, and how to get it into our cells.
Finally, in 2018, the FDA approved patisiran (Onpattro®), a drug that treats a rare nerve disease. The method patisiran uses to deliver RNA (lipid nanoparticles) paved the way for Moderna and Pfizer-BioNTech to use it in their COVID vaccines.
In the U.S., COVID vaccines were free for the first couple of years after they were introduced. That’s because the U.S. government funded programs to make sure anyone who wanted to could get vaccinated. Those programs will end sometime in 2023. Future COVID vaccinations might not cost you anything if your insurance or your employer covers them.
A note from Cleveland Clinic
Scientists were able to develop COVID vaccines in record time thanks to decades of research in mRNA technology. Before the vaccines, millions of people were seriously ill or died of COVID-19. Hospitals were overflowing. Since the FDA authorized the first COVID vaccines in late 2020, over 270 million people (or 81% of the U.S. population) have received at least one dose. This has prevented millions of deaths and hospitalizations. The technology they use is promising for developing future vaccines quickly.
Last reviewed by a Cleveland Clinic medical professional on 06/02/2023.
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