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
On this page
Key takeaways
- COVID-19 vaccines use the code for a part of the virus to help train and strengthen your immune system to recognize and help fight the virus, even if you¡¯ve been previously infected.
- COVID-19 vaccines have been updated to closely match the recent changes in the virus that causes COVID-19.
- Some common COVID-19 vaccine side effects are soreness at the injection site, tiredness, headaches, or fever. Side effects can vary from person-to-person. Serious reactions from vaccines are rare, but they may happen.
- COVID-19 vaccines go through extensive research, analysis, and transparent reporting to evaluate their safety and effectiveness. Their use is recommended by the Centers for Disease Control and Prevention and many professional organizations.
What is the 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 the code for a part of the virus to train your immune system to recognize and help protect against COVID-19 infection and disease:
- 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 a COVID-19 vaccine or vaccinated against COVID-19 happens in two stages: an initial vaccine dose or series of doses, and then updated vaccines as recommended over time. For people with a weakened immune system, more doses may be needed compared to other people. 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.
It¡¯s safe and recommended for children, teens, and adults to get all age-appropriate doses of vaccines simultaneously during a doctor¡¯s visit, also known as coadministration, as long as there¡¯s no medical reason not to.
As the virus that causes COVID-19 continues to change, updates vaccines are designed to help your body recognize more recently circulating strains. (Similar to how the annual flu shot is reformulated each year.) This approach 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 fight the virus
Not quite
Correct answer: Your immunity wanes over time, and strains change; the latest vaccine helps your immune system recognize and fight the virus
Who should get the COVID-19 vaccine?
Though eligibility may vary by vaccine manufacturer, current guidance is that those 65 or older, and those 6 months to 64 years old with at least one risk factor for severe COVID-19, may be eligible to receive a COVID-19 vaccine. Talk to your doctor or pharmacist for the latest information.
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.
Not recently vaccinated:
Since immunity and protection from infection or vaccination gets weaker over time, getting an updated COVID-19 shot can help protect you.
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, updated doses help keep your immune system one step ahead.
COVID-19 vaccination safety
COVID-19 vaccines have gone through extensive studies that have demonstrated their safety and efficacy. The first studies were performed prior to approval for use in the United States. Since the first sets of studies, many more studies have demonstrated both effectiveness in preventing serious illness, and safety.
Adults may experience side effects after vaccination¡ªsuch as soreness at the injection site, tiredness, headaches, or fever¡ªbut those side effects are usually short-lived. These reactions are typical and indicate your immune system is building up protection.
Further, studies have demonstrated that COVID-19 may cause serious disease and death. Importantly, more than 1,200,000 Americans have died from COVID-19.
Rare cases of inflammation of the heart (myocarditis and pericarditis) have been reported after an mRNA vaccination, most commonly in young males 12 to 24 years old. Some individuals have required intensive care support, but available data suggest that individuals typically have symptoms improving or disappearing within a few days with medicines used to reduce inflammation. Importantly, COVID-19 infection also may cause inflammation of the heart, and COVID-19 infection has been associated with an increased risk of heart attacks and strokes.
Serious reactions from vaccines are rare, but they can happen¡ªand 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¡¯re feeling afterward. If something feels off, don¡¯t wait¡ªreach out to your healthcare provider. When in doubt, it¡¯s always better to check in.
On the other hand, severe infection from the SARS-CoV-2 virus carries substantial risk, including an increased chance of hospitalization, development of Long COVID-19, and death.
Staying up-to date with recommended COVID-19 vaccine doses is one of the best ways to help protect against serious disease from COVID-19.
65+
High-priority group
How often should you get the COVID-19 vaccine?
As the virus that causes COVID-19 evolves, so does the vaccination plan. Though eligibility may vary by vaccine manufacturer, current guidance is that those 65 or older, and those 6 months to 64 years old with at least one risk factor for severe COVID-19, may be eligible to receive a COVID-19 vaccine. Talk to your doctor or pharmacist for the latest information.
How long does it take to get protection from a COVID-19 vaccine?
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. That means you won't reach peak protection against COVID-19 immediately, but your immune system will be actively preparing defenses in the background.
COVID-19 vaccine: help protect yourself against COVID-19
COVID-19 vaccination remains one of the most effective ways to help protect yourself from severe illness and complications that can be caused by the SARS-CoV-2 virus. If you're not sure if you're due for a vaccine, talk to your healthcare provider or check your eligibility at VaxAssist.com and schedule your appointment today.
Frequently asked questions
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.
References
World Health Organization. COVID?19 vaccines [dashboard]. WHO. https://data.who.int/dashboards/covid19/vaccines
Centers for Disease Control and Prevention. How COVID?19 vaccines work. CDC. Published April 15, 2024. https://www.cdc.gov/covid/vaccines/how-they-work.html
World Health Organization. COVID?19 advice for the public: Getting vaccinated. WHO. Published October 8, 2024. https://www.who.int/emergencies/diseases/novel-coronavirus-2019/covid-19-vaccines/advice
Centers for Disease Control and Prevention. Staying up to date with COVID?19 vaccines. CDC. Published June 6, 2025. https://www.cdc.gov/covid/vaccines/stay-up-to-date.html
Link?Gelles R. Effectiveness of COVID?19 vaccines. CDC ACIP; October 23, 2024. https://www.cdc.gov/acip/downloads/slides-2024-10-23-24/04-COVID-Link-Gelles-508.pdf
Englund J, Feldstein LR, Ruffin J, Wiegand RE, et?al. Effectiveness of mRNA COVID?19 vaccines and hybrid immunity in [study]. J Infect Dis. 2025;231(4): e743¨Ce753. https://pubmed.ncbi.nlm.nih.gov/39774936/
Beladiya J, Kumar A, Vasava Y, Parmar K, Patel D, Patel S, ¡ Patel CA. Safety and efficacy of COVID?19 vaccines: A systematic review and meta?analysis of controlled and randomized clinical trials. Rev Med Virol. 2024;34(1): e2507. https://pubmed.ncbi.nlm.nih.gov/38282394/
Centers for Disease Control and Prevention. Coronavirus disease 2019 (COVID?19) vaccine safety. CDC. Published January 31, 2025. https://www.cdc.gov/vaccine-safety/vaccines/covid-19.html
Tu M, Chiang SH, Bender R, Wong DTW, Strom C. The kinetics of COVID?19 vaccine response in a community?vaccinated population [preprint]. medRxiv. Published September 28, 2021. https://pmc.ncbi.nlm.nih.gov/articles/PMC8491861/
Pilecki MW, Bolesta E, Lewandowska M, ?urawska A, Witkowski J. Emerging heterologous mRNA?based booster strategies within the global COVID?19 vaccine rollout. Vaccines (Basel). 2022;10(11):1823. Published November?2022. https://pmc.ncbi.nlm.nih.gov/articles/PMC9980456/
Allayee H, Rathore R, Keever B, et al. COVID-19 Is a Coronary Artery Disease Risk Equivalent and Exhibits a Genetic Interaction With ABO Blood Type. Arterioscler Thromb Vasc Biol. 2024. https://atvb.ahajournals.org/doi/epub/10.1161/ATVBAHA.124.321001