How Does the COVID-19 Vaccine Work?

This page is no longer updated and will be removed in June 2023. For updated COVID-19 information and health materials, please go to our COVID-19 Vaccine page.

Vaccines help our immune system fight future infections. COVID-19 vaccines train our bodies to develop defenses to the disease without having to get sick.

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How Vaccines Work In Your Body

Benefits of Getting Vaccinated

The COVID-19 vaccines help protect people in these ways:

  • They can greatly reduce your chance of getting seriously ill, even if you get COVID-19
  • Completing the vaccine series reduces your chances of hospitalization and lowers your risk of dying from COVID-19
  • They are highly effective at preventing COVID-19
  • They add to the number of people in the community who are protected from getting COVID-19 — making it harder for the disease to spread
  • Experts continue to conduct more studies about the ability of the vaccine to keep people from spreading the virus to others

How COVID-19 Vaccines Work

MRNA vaccines

Two of the vaccines available are messenger RNA (mRNA) vaccines. MRNA vaccines teach your cells to produce a harmless piece of the coronavirus spike protein that then triggers an immune response to build antibodies. In this way, you build immunity to COVID-19 without getting the illness. Once it does its job, the mRNA quickly breaks down and the body clears it away in a few days.

It typically takes about two weeks after the second dose to become fully protected.

Viral vector vaccines

One of the COVID-19 vaccines authorized by the FDA is a viral vector vaccine. Vector vaccines contain a weakened version of a virus—a different virus than the one that causes COVID-19—that teaches your cells to make a harmless piece of coronavirus spike protein. Your immune system sees that the protein shouldn't be there, which triggers your body to make antibodies that remember how to fight that virus if you are infected with COVID-19 in the future.

The vector vaccine we have available is a one dose vaccine. It typically takes about two weeks after you get the vaccine to become fully protected.

Protein subunit vaccines

One of the COVID-19 vaccines authorized by the FDA is a protein subunit vaccine. Protein subunit vaccines contain pieces of the virus (proteins) that causes COVID-19 (made without using any live virus) with an additive intended to help the vaccine work better in the body. Once your immune system knows how to respond to the spike protein, it will then be able to respond quickly to the actual virus and protect you against COVID-19. Subunit vaccines cannot cause infection with the virus that causes COVID-19 and do not interact with our DNA.

The viral subunit vaccine available is a two-dose vaccine. It typically takes about two weeks after the second dose to become fully protected.

Sometimes vaccination can cause mild fever or cold-like symptoms, but these are not harmful. To learn more about the FDA-authorized vaccines, visit the Safety and Effectiveness page.

More information about these vaccines is available on the CDC website.

Frequently Asked Questions About the Vaccine

What does it mean when a vaccine is fully licensed?

For full approval, the FDA evaluates data over a longer period of time than for an emergency use authorization. For the vaccine to be given full approval, the data must show a high level of safety, effectiveness, and quality control in vaccine production. The purpose of emergency use authorization is to ensure that people can get lifesaving vaccines prior to a longer-term analysis of data. However, EUA still requires a very thorough review of clinical data—just over a shorter period of time.

How do we know the vaccines are safe?

To make sure that COVID-19 vaccines are safe, CDC expanded and strengthened the country’s ability to monitor vaccine safety. As a result, vaccine safety experts can monitor and detect issues that may not have been seen during the COVID-19 vaccine clinical trials.

How will a COVID-19 vaccine work in my body?

The COVID-19 vaccine teaches your immune system to recognize the coronavirus. When you get the vaccine, your immune system makes antibodies (“fighter cells”) that stay in your blood and protect you in case you are infected with the virus. You get protection against the disease without having to get sick.

When enough people in the community can fight off the coronavirus, it has nowhere to go. This means we can stop the spread quicker and get a little closer to ending this pandemic.

How are COVID-19 vaccines made?

This short video explains how COVID vaccines are made.

What is an mRNA vaccine?

A messenger RNA, or mRNA vaccine is a new type of vaccine. mRNA vaccines teach your cells how to make a harmless piece of the “spike protein.” The spike protein is what you see on the surface of the coronavirus. Your immune system sees that the protein doesn't belong there and your body will start to build an immune response and make antibodies. This is similar to what happens when we “naturally” get a COVID-19 infection. Once it does its job, the mRNA quickly breaks down and the body clears it away in a few days.

Although we have used mRNA for other types of medical and veterinary care in the past, creating vaccines using this method is a huge leap forward in science and may mean future vaccines can be created more easily.

You may read more about how mRNA vaccines work on CDC's website, or watch this video from Dr. Paul Offit at the Children's Hospital of Philadelphia.

What is a viral vector vaccine?

This type of vaccine uses a weakened version of a different virus (the “vector”) that gives your cells instructions. The vector enters a cell and uses the cell's machinery to create a harmless piece of the COVID-19 spike protein. The cell displays the spike protein on its surface, and your immune system sees that it doesn't belong there. Your immune system will start to make antibodies and activate other immune cells to fight off what it thinks is an infection. Your body learns how to protect you against future infection with COVID-19, without you having to get sick.

What is a protein subunit vaccine?

One of the COVID-19 vaccines authorized by the FDA is a protein subunit vaccine. Protein subunit vaccines contain pieces of the virus (proteins) that causes COVID-19 (made without using any live virus) with an additive intended to help the vaccine work better in the body. Once your immune system knows how to respond to the spike protein, it will then be able to respond quickly to the actual virus and protect you against COVID-19. Subunit vaccines cannot cause infection with the virus that causes COVID-19 and do not interact with our DNA.

What is an adjuvant?

The adjuvant in the Novavax vaccine is the additive intended to help strengthen the body’s immune response.

What types of symptoms are normal after receiving the vaccine?

Like other routine vaccines, the most common side effects are a sore arm, fatigue, headache, and muscle pain.

These symptoms are a sign that the vaccine is working. In the Pfizer, Moderna and Novavax trials, these side effects occurred most often within two days of getting the vaccine, and lasted about a day. Side effects were more common after the second dose (of the initial two-dose series) than the first dose. In the Johnson & Johnson clinical trials, side effects lasted an average of one to two days. For all three vaccines, people over age 55 were less likely to report side effects than younger people.

You may see some rumors about untrue side effects online or on social media. Make sure any time you see a claim about a side effect that you check the source of that claim. This video can teach you more about how to figure out if a claim online is true or not.

What ingredients are in the vaccines?

The ingredients in the COVID-19 vaccines are pretty typical for vaccines. They contain the active ingredient of mRNA or modified adenovirus along with other ingredients like fats, salts, and sugars that protect the active ingredient, help it work better in the body, and protect the vaccine during storage and transport.

The Novavax COVID-19 vaccine is a protein subunit-based vaccine that contains an additive, along with fats and sugars to help the vaccine work better in the body. This vaccine does not use mRNA.

None of the vaccines contain human cells (including fetal cells), the COVID-19 virus, latex, preservatives, or any animal by-products including pork products or gelatin. The vaccines are not grown in eggs and do not contain any egg products.

See this Q&A; webpage from the Children's Hospital of Philadelphia for more information about ingredients. You can also find the full ingredients lists in the Pfizer, Moderna, Novavax and Johnson & Johnson fact sheets.

Does the Johnson & Johnson vaccine contain fetal tissue?

The Johnson & Johnson COVID-19 vaccine was created using the same technology as many other vaccines. It does not contain parts of fetuses or fetal cells.

In this video, Dr. Paul Offit addresses fetal cells and COVID-19 vaccines:

Can I get the COVID-19 vaccine if I'm pregnant, lactating or planning to become pregnant?

Yes, data show that COVID-19 vaccines are safe during pregnancy. The Centers for Disease Control and Prevention (CDC), American College of Obstetricians and Gynecologists (ACOG), and Society for Maternal-Fetal Medicine (SMFM) recommend the COVID-19 vaccine for people who are pregnant, lactating, or planning to get pregnant.

Some studies show that if you are vaccinated, your baby may even get antibodies against COVID-19 through pregnancy and lactation. Unvaccinated pregnant people who get COVID-19 are at increased risk of severe complications like preterm birth or stillbirth. In addition, people who get COVID-19 while pregnant are two to three times more likely to need advanced life support and a breathing tube.

For more resources about getting the COVID-19 vaccine while pregnant and breastfeeding, please see up to date information on the One Vax, Two Lives website.

What is an Emergency Use Authorization (EUA)?

An EUA allows the FDA to make a product available during a declared state of emergency before it has a full license. Any EUA granted by the FDA is further vetted by the Scientific Safety Review Workgroup, as part of the Western States Pact.

What is the Western States Pact?

In October, 2020 Washington joined Oregon, Nevada, Colorado and California to form the Western States Scientific Safety Review Workgroup (Western States Pact) to review the safety and efficacy of COVID-19 vaccines after they were authorized by the FDA. This workgroup provided another layer of expert review around vaccine safety.

The panel included experts appointed by all member states, and nationally recognized scientists with expertise in immunization and public health. To act in close coordination and collaboration with western states when COVID-19 was declared a health emergency, this panel reviewed all publicly available data concurrently with federal reviews for the four vaccines we currently have available in Washington state. As we move into a new recovery phase of the pandemic, the Western States Pact has disbanded and all vaccine use authorization in western states will be determined by the FDA and vaccine recommendations will come from the CDC.

Read the findings of the Western States Scientific Safety Review Workgroup:

Is it COVID-19 or a vaccine reaction?

After getting a COVID-19 vaccine, you may have some side effects. These are normal signs that your body is building protection against COVID-19. Your arm may hurt where you got your shot or you may have redness or swelling. You may be tired or have a headache, muscle pain, chills, fever, or nausea. They may affect your ability to do daily activities but should go away in a few days. Some people have no side effects. Learn more about possible side effects after getting a COVID-19 vaccine.

If the symptoms do not go away after a few days, you should seek medical advice. If there’s a possibility you have COVID-19 or were exposed, please stay away from others as a precaution. If you experience a medical emergency after getting the COVID-19 vaccine, call 9-1-1 immediately.

Can I get the COVID-19 vaccine if I've had an allergic reaction to a vaccine in the past?

The vaccine should not be given to people with a known history of severe allergic reaction, such as anaphylaxis, to a previous dose of an mRNA or viral vector vaccine, or to any ingredient of the Pfizer-BioNTech/Comirnaty, Moderna/Spikevax, Novavax or Johnson & Johnson–Janssen COVID-19 vaccines.

People who have had a severe allergic reaction to other vaccines or injectable therapies may still be able to receive the vaccine. However, providers should do a risk assessment and counsel them about potential risks. If the patient decides to get the vaccine, the provider should observe them for 30 minutes to monitor for any immediate reactions.

The Advisory Committee on Immunization Practices (ACIP) recommends that providers observe all other patients for at least 15 minutes after receiving the vaccine to monitor for an allergic reaction. See ACIP's interim clinical considerations for vaccines for more information.

What happens if I get sick after getting the COVID-19 vaccine?

It's normal to have some side effects after getting the vaccine. This can be a sign that the vaccine is working. If you experience a medical emergency after getting the COVID-19 vaccine, call 9-1-1 immediately.

If you get sick after getting the vaccine, you should report the adverse event to the Vaccine Adverse Event Reporting System (VAERS). An “adverse event” is any health problem or side effect that happens after a vaccination.

For more information about VAERS, see "What is VAERS?" below.

What is VAERS?

VAERS is an early warning system led by the Centers for Disease Control and Prevention (CDC) and the Food and Drug Administration (FDA). VAERS can help detect problems that may be related to a vaccine.

Anyone (health care provider, patient, caregiver) can report possible adverse reactions to VAERS.

There are limits to the system. A VAERS report does not mean the vaccine caused the reaction or outcome. It only means that the vaccination happened first.

VAERS is set up to help scientists notice trends or reasons they should investigate a possible problem. It is not a list of verified outcomes of vaccination.

When you make a report to VAERS, you help the CDC and the FDA identify possible health concerns and make sure vaccines are safe. If any issues arise, they will take action and notify health care providers about potential issues.

What is a COVID-19 variant?

Viruses mutate (change) as they spread from person to person. A ‘variant' is a mutated strain of virus. Some variants disappear over time and some continue to spread in communities.

The Centers for Disease Control and Prevention (CDC) identifies the virus variants that are concerning. Currently, several variants are concerning because they spread quickly and more easily, causing more COVID-19 infections.

Does the COVID-19 vaccine work against variant strains?

COVID-19 vaccines are working well to prevent severe illness, hospitalization, and death, even against variants. However, public health experts are seeing reduced protection against mild and moderate COVID-19 illness, especially among-high risk populations.

The updated boosters were created to help boost immunity and provide better protection from the omicron variant. It's important to get all recommended doses available for the best protection.

Vaccination is the best way to protect you, your loved ones, and your community. High vaccination coverage will reduce the spread of the virus and help prevent new virus variants from emerging.

Visit our pages on the COVID-19 variants and vaccine breakthrough for more information.

Why should I get the COVID-19 vaccine if most people survive having the disease?

Death is not the only risk from having COVID-19. Many people who get COVID-19 only have mild symptoms. However, the virus is extremely unpredictable, and we know some COVID-19 variants are more likely to make you really sick. Some people can get very sick or die from COVID-19, even young people with no chronic health conditions. Others, known as "COVID long-haulers" may get symptoms that last for months and affect their quality of life. We also don't know yet all the long-term effects of COVID-19 since it's a new virus. Getting vaccinated is our best protection against the virus. Even if you're young and healthy, you should get a COVID-19 vaccine.

If I get a COVID-19 vaccine, do I still need to take other precautions?

Yes, even if you get vaccinated, you're required to wear a mask in some public indoor settings. Find more information on our Masks and Face Coverings FAQ page. We also recommend you wash your hands often, stay six feet apart, and limit gatherings.

The COVID-19 vaccines work well, but they are not 100% effective. Some people may get COVID-19 even if they've been vaccinated. With the rise of more transmissible variants, it's important that all people take precautions such as wearing masks to reduce transmission of the virus.

What is the difference between vaccination and immunity?

Natural immunity from infection does offer some level of immunity against reinfection but it is important to stress that initial infection among unvaccinated persons increases risk for serious illness, hospitalization, and death. While some people may develop antibodies after COVID-19 infection, others may not. For those that develop some immunity after infection, there is no way to tell how strong that protection is, how long it will last or even which variant the immunity is for.

Because we cannot rely on natural immunity to prevent reinfection or severe illness from COVID-19, being up to date on vaccination remains the best protection and primary strategy to prevent SARS-COV-2 infections, associated complications, and onward transmission.

For more information on immunity and COVID-19, please review these helpful resources.