Understanding COVID-19 vaccines
Learn what an updated COVID-19 vaccine is, who should get it, and why staying up to date still matters.
Dr. David Weber · August 13, 2025
6 min read
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Key takeaways
- COVID-19 vaccines can help train and strengthen your immune system to recognize and help fight the virus, even if you’ve already been infected.
- COVID-19 vaccines have been updated to more closely match currently circulating strains of the virus that causes COVID-19.
- Some common COVID-19 vaccine side effects are pain, swelling, and redness at the injection site, tiredness, headache, joint or muscle pain, or fever. Side effects can vary from person-to-person. Serious reactions from vaccines are rare, but they can happen.
- COVID-19 vaccines go through extensive research, analysis, and transparent reporting to evaluate their safety and effectiveness. Their use is recommended by the US Centers for Disease Control and Prevention and many professional organizations.
- Individuals 65 years of age or older, or 6 months to 64 years of age with at least one underlying condition that puts them at high risk of severe outcomes from COVID-19 may be eligible for COVID-19 vaccination.
- CDC recommends talking to your doctor or pharmacist about the benefits and risks of COVID-19 vaccination.
COVID-19 vaccines were first authorized in 2020, some were approved in 2021, and since then billions of doses have been administered worldwide, reducing severe illness, hospitalization, and death as a result of COVID-19.
But you might still wonder: “What is a COVID-19 vaccine? Do I need one—or another dose?”
What is a COVID-19 vaccine?
COVID-19 vaccines train your immune system to recognize and help fight the SARS-CoV-2 virus that causes COVID-19. Unlike some vaccines that contain weakened viruses, COVID-19 vaccines use different approaches to train your immune system to recognize and help protect against COVID-19 infection and disease. For example, mRNA vaccines use the code for a part of the COVID-19 virus, while protein subunit vaccines contain a small piece of the COVID-19 virus called the spike protein.
- mRNA vaccines: These vaccines contain a small piece of mRNA that codes for a protein found on the surface of the virus that causes COVID-19. The vaccine cannot cause COVID-19 infection. It helps your immune system learn to recognize and help fight the virus if you're later exposed.
- Protein subunit vaccines: Contain fragments (small pieces) of the surface coat of the virus that activate your immune system and train your body to help fight against COVID-19, without the risk of infection.
Getting vaccinated against COVID-19 happens in two stages: an initial vaccine dose or series of doses, and then an updated vaccine as recommended by the CDC. This process helps to first activate your immune system, and the follow-up doses help renew and strengthen that protection and help protect against newer strains, since immune response decreases over time and the virus keeps changing.
Some people who are immunocompromised (have a weakened immune system) are more likely to get very sick with COVID-19. The immune response to COVID-19 vaccination may not be as strong as people who are not immunocompromised. Ask your healthcare provider if you may have different recommendations for COVID-19 vaccines.
As the virus that causes COVID-19 continues to change, updated vaccines are designed to help your body recognize more recently circulating strains. This helps train and strengthen your immune system.
Test your knowledge
Why is it still important to get a COVID-19 vaccine?
Correct
Your immunity wanes over time, and strains change; the latest vaccine helps your immune system recognize and help fight the virus
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Correct answer: Your immunity wanes over time, and strains change; the latest vaccine helps your immune system recognize and help fight the virus
Who should get a COVID-19 vaccine?
Individuals 65 years of age or older, or 6 months to 64 years of age with at least one underlying condition that puts them at high risk of severe outcomes from COVID-19 may be eligible for COVID-19 vaccination. CDC recommends talking to your doctor or pharmacist about the benefits and risks of vaccination.
Previously infected individuals:
Even if you've had COVID-19, research shows that vaccination can help provide additional protection when combined with immunity from previous infection, compared to either vaccination or immunity from previous infection alone. Talk to your doctor or pharmacist about the best time to get a COVID-19 vaccine.
Not recently vaccinated:
Immunity after a COVID-19 infection and protection from a COVID-19 vaccine decreases with time. An updated COVID-19 vaccine can help protect against currently circulating strains.
Pregnant women:
If you are pregnant or were recently pregnant, you may be more likely to get very sick from COVID-19. The COVID-19 vaccine may help protect women who are pregnant, breastfeeding, trying to get pregnant now, or might become pregnant in the future.
As the virus changes, an updated vaccine dose(s) can help keep your immune system ready.
COVID-19 vaccination safety and efficacy
COVID-19 can lead to severe outcomes, such as the need to be hospitalized or receive intensive care, require a ventilator to breathe, and/or death. More than 1.2 million Americans died from COVID-19 from 2020 through July 2024.
Some people are more likely than others to get very sick if they get COVID-19, including people who are older, are immunocompromised (have a weakened immune system), have certain disabilities, or have underlying health conditions or risk factors.
Vaccination can help protect eligible individuals against COVID-19 and severe illness.
COVID-19 vaccines have gone through extensive studies that have demonstrated their safety and efficacy. Randomized controlled studies were started in 2020, prior to authorization and approval in the United States. Additional studies over the years have continued to support effectiveness and safety of COVID-19 vaccines.
Pain, swelling, and redness where the shot is given, fever, tiredness (fatigue), headache, chills, muscle pain, joint pain, nausea, vomiting, and swollen lymph nodes can happen after COVID-19 vaccination.
Rare cases of myocarditis (inflammation of the heart muscle) and pericarditis (inflammation of the lining outside the heart) have been reported after an mRNA vaccination, most commonly in young males 12 to 24 years old.
Tell your healthcare provider if you have had an allergic reaction after a previous dose of COVID-19 vaccine, or have any severe, life-threatening allergies.
Serious allergic reactions from vaccines are rare, but they can happen—and usually show up within a few minutes to a few hours after getting the shot, though some can take up to a week or more to appear. That’s why it’s a good idea to pay attention to how you feel afterward. If something feels off, don’t wait—reach out to your healthcare provider. When in doubt, it’s always better to check in.
Staying up-to date with recommended COVID-19 vaccine doses is one of the best ways to help protect against getting severely ill from COVID-19.
65+
High-risk
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How long does it take for COVID-19 vaccines to work?
Once you receive a COVID-19 vaccine, your body begins to build protection within days. For most adults who’ve already received a COVID-19 vaccine, a protective response develops around 10 to 14 days after the vaccine.
COVID-19 vaccine: help protect yourself against COVID-19
COVID-19 vaccination remains one of the most effective ways to help protect yourself from getting severely ill. If you're not sure if you're due for a vaccine, talk to your doctor or pharmacist or check your eligibility at VaxAssist.com and schedule your appointment today.
This article was medically reviewed by Dr. David Weber, a compensated expert engaged by Pfizer to ensure the information provided is medically accurate and current. The content is for educational purposes only and is not a substitute for professional medical advice.
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