COVID-19 Vaccines

Medically Reviewed by Jabeen Begum, MD on September 04, 2025
9 min read

The COVID-19 vaccine helps train your immune system to fight off coronavirus disease (COVID) caused by the SARS-CoV-2 virus. The coronavirus can cause serious illness and potentially death. But vaccination is one of the best ways to stop it.

Your risk of serious coronavirus illness is similar to or higher than that of the flu, depending on your age and medical conditions. COVID-19 is now seasonal, like colds and influenza. Historically, we would use a vaccine to help control it. But this fall, the COVID-19 vaccine may be limited.

Find out what the vaccine does, how it's developed, the types, and who can get COVID-19 vaccines this season.

A COVID vaccine is a biological medicine that trains your immune system to make antibodies against the virus. Antibodies are tools to help you fight off and destroy COVID infection to prevent serious, potentially life-threatening illness. 

The vaccine contains harmless pieces of the COVID virus or bits of its genetic material. These tiny, man-made germ bits help your body make antibodies against the virus that causes COVID. Then, if you come in contact with COVID again, your immune system can kill off any infection. 

If you're exposed to COVID right before or right after you get vaccinated, you could still get sick. That's because it takes a few weeks for your body to make those antibodies. But once the antibodies are made, you'll have immunity so you don’t get sick or your illness is much milder. 

How do COVID vaccines train your immune system?

There are two main ways COVID vaccines boost your immune response: using a man-made protein or with a bit of the viral genetic code.

RNA vaccines. This type of COVID shot carries a piece of the genetic code into your body – usually messenger RNA. It's a temporary "message" telling your cells to make a harmless germ protein. This trains your immune system to make antibodies before the message is broken down and discarded.

Recombinant protein vaccine. This vaccine contains a harmless piece of the COVID virus called the spike protein. This spike protein is made in the lab and trains your immune system to make antibodies against COVID.

Experts developed the COVID vaccine at an unparalleled pace. The pandemic triggered thousands of researchers to test over 100 forms of the vaccine, round-the-clock. But before it's used widely, the vaccine must prevent COVID without causing other problems. In general, a vaccine develops along this timeline:

Exploratory stage

Experts start lab research (often two to four years) to find how to treat or prevent the disease.

Preclinical stage

Scientists use animals (mice or rats) and lab tests to learn how the vaccine works. In the 1990s, researchers were using mRNA influenza vaccines to treat rats. This stage usually lasts one to two years, but many vaccines don't make it past this point. If the vaccine does, it moves into clinical testing.

Clinical development

This three-phase process tests the vaccine in humans. 

  • Phase 1 tests vaccine safety in fewer than 100 people, usually lasting one to two years. 
  • Phase 2 takes at least two years to test the right vaccine amount on several hundred people. 
  • Phase 3 lasts three or four years to make sure the vaccine can prevent illness in thousands of people without causing problems. 

COVID-19 began in 2019 with the first COVID vaccines approved in the U.S. in 2020.

Regulatory review and approval

Scientists with the FDA and CDC go over the data from the clinical trials and sign off.

Manufacturing

The vaccine is made in large amounts for sale. The FDA inspects the factory and approves drug labels.

Quality control 

Your doctor, scientists, and government agencies keep tabs on how the vaccine is made and on people who get it. They want to make sure the vaccine keeps working safely.

There are four vaccines approved in the United States. Three are mRNA vaccines, and one is a recombinant protein vaccine. 

Pfizer-BioNTech (Comirnaty)

The mRNA vaccine carries a man-made genetic message to tell your cells to make the COVID spike protein. Your immune system can make antibodies and kill off any cells making this spike protein, giving you some immunity against it.

Who is the vaccine approved for? This single-dose vaccine is approved for anyone 65 years and older. It’s also available for people 5 to 64 years old with at least one medical condition that puts them at risk of serious COVID-19 illness.

Moderna (Spikevax)

The vaccine carries the man-made messenger RNA instructions for the COVID spike protein. After vaccination, your body can make antibodies to protect you from the coronavirus.

Who is the vaccine approved for? The single-dose vaccine is approved for anyone 65 years and older. You can get this vaccine at least two months after your last COVID shot.

It’s also available for people 6 months old to 64 years old with at least one medical condition that puts them at risk of serious COVID-19 illness. If you haven’t been vaccinated for COVID, you’ll get two doses, a month apart. Otherwise, you can get the vaccine at least two months after your last COVID shot.

Moderna (mNexspike)

The vaccine carries two man-made RNA messages to instruct your cells to make two pieces of the COVID spike protein. This trains your immune system to make antibodies and protect against COVID infection.

Who is the vaccine approved for? A single dose is approved for anyone 65 years and older, two months after your last COVID vaccine. But people 12 to 64 years old must have at least one medical condition that puts them at risk of serious COVID-19 illness to get this shot, at least two months after their last COVID shot.

Novavax (Nuvavax)

This COVID vaccine contains a man-made spike protein grown in insect cells to help your immune response. The shot also contains a compound called matrix-M. Matrix-M is called an adjuvant, a compound that boosts your immune response. It’s made from the bark of the South American soapbark tree.

Just like the hepatitis B and influenza vaccines, this COVID vaccine uses protein subunits to train your immune system. This can help protect you against future COVID infection. The Novavax shot is like other vaccines used in the U.S. for more than 30 years. And if you prefer a vaccine built on that type of technology, this vaccine is a good option.

Who is the vaccine approved for? A single dose is approved for people 65 years and older. The vaccine is also for people 5 to 64 years old with at least one medical condition that raises the risk of serious COVID-related illness. You can get this shot at least two months after your last COVID vaccine.

Does the CDC recommend COVID vaccination?

Yes, the CDC says that everyone 65 years and older should get a COVID vaccine each year. But if you’re 6 months to 64 years old, talk with your doctor about it. 

Keep in mind that you’ll need to be eligible to get the vaccine. That means you may need to have at least one medical condition such as:

  • Asthma or chronic lung problems
  • Smoke or substance use disorder
  • Cancer
  • Cerebrovascular disease
  • Chronic kidney disease
  • Chronic liver disease
  • Diabetes
  • Heart problems
  • Blood conditions (such as hemophilia or sickle cell)
  • HIV or tuberculosis
  • Mental health conditions (such as ADHD and mood disorders)
  • Brain conditions (such as dementia, epilepsy, or Parkinson's disease)
  • Obesity or physical inactivity
  • Pregnancy or recent pregnancy
  • Immunodeficiency or transplant

Can you get the flu and COVID-19 vaccines at the same time?

Yes. The CDC says you can get your flu and COVID-19 vaccines at the same time. Experts say that your body’s process to build antibodies is the same – alone or with the flu vaccine. Flu season in the U.S. typically lasts from October to May.

As with any medication, you may get some side effects. Many of the side effects can mimic a very mild COVID infection, and after vaccination, you can commonly get:

  • Fatigue
  • Headache
  • Achy muscles
  • Fever
  • Pain where the needle went in

COVID vaccine complications

There are two main complications of the COVID vaccine: an allergic reaction and inflammation around your heart. Allergic reactions are very rare and typically happen within the first 15 minutes after you get your COVID vaccine.

Inflamed heart muscle (myocarditis) and inflamed heart lining (pericarditis)

You may have a higher risk of myocarditis and pericarditis if you’re male and 12 to 24 years of age. Even though a few people need hospital care, most recover with treatment after a few days.

When to call the doctor

In rare cases, you can get symptoms of a serious allergic reaction. Almost 1 out of every 200,000 people who get a COVID vaccine get anaphylaxis reactions. If you have any of these symptoms of this life-threatening condition, get medical care right away:

  • Hives
  • Swelling in your face, lips, tongue, or throat
  • Itching
  • Rash

The COVID-19 vaccine can train your immune system to make antibodies so you can fight off the virus and prevent potentially life-threatening coronavirus disease (COVID). The Comirnaty, Spikevax, and mNexspike COVID vaccines contain genetic material, but the Nuvavax vaccine has man-made protein to boost your immune response. The CDC recommends the COVID vaccine for anyone 65 years and older, but if you’re younger, talk to your doctor about your options.

What does it mean to be fully vaccinated for COVID?

If you’re eligible for the COVID vaccine and you’ve never had the vaccine, you’ll get:

  • Two Spikevax doses, one month apart if you're 6 to 23 months old
  • One Spikevax dose if you're 2 years and older
  • One Comirnaty dose if you're 5 years or older
  • One mNexspike dose if you're 12 years and older
  • One Nuvavax dose if you're 12 years or older

If you’re eligible and you’ve had the COVID vaccine before, at least two months after your last dose, you’ll get:

  • One Spikevax dose if you're 2 years or older
  • One Comirnaty dose if you're 5 years or older
  • One mNexspike dose if you're 12 years or older
  • One Nuvavax dose if you're 12 years or older

Can I volunteer for COVID vaccine clinical trials?

Yes. If you're interested in volunteering for a vaccine trial, you can call, email, or visit the website for:

  • Prevent COVID. The COVID Prevention Network is a government-funded site (National Institute of Allergy and Infectious Diseases) coordinated by the Fred Hutchinson Cancer Center in Seattle. Many research centers are using this site to find volunteers.
  • ClinicalTrials.gov. This is a government database of public and private clinical studies done worldwide. The site also offers considerations for joining a clinical trial.
  • Research centers. CenterWatch is a research hub that can help you find clinical trials.
  • Individual hospitals. Some hospitals like Kaiser Permanente and Penn Medicine may provide clinical trials for COVID, too.
  • Local universities. Research universities also offer information on COVID-19 vaccine trials, such as the University of Alabama at Birmingham, Saint Louis University, and University of California, Davis and San Diego.

How long does the COVID vaccine last?

COVID is seasonal, like influenza. If you’re 65 or older, the CDC recommends you get your COVID vaccine each year. But if you’re 6 months old to 64 years old, talk with your doctor to see if the vaccine is available to you.

Do I need a COVID vaccine card?

Your vaccine card proves you’ve had the COVID vaccine. You'll get a card the first time you're vaccinated, but many providers keep digital copies. Contact your health care provider or local health department to ask for your digital vaccine card.

Take a picture of it or print it out. That way, you’ll have your own copy, too. The CDC stopped making the COVID-19 vaccination cards in October 2023.