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What to know about vaccine side effects
Vaccines are a safe and effective way to help your body prevent illness, but side effects are possible. Learn more about the safety and risks of vaccines.
Dr. David Weber · August 13, 2025
6 min read
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Key takeaways
Vaccines are safe and effective, but side effects may occur. They’re usually a sign your immune system is doing its job, but some may require further care.
- Most side effects are mild and temporary, like local redness at the site of the vaccine shot or fatigue.
- Serious side effects are rare and should be closely followed.
- Understanding the difference between expected minor side effects and serious reactions can help ease anxiety.
- Special considerations exist for older adults and pregnant women.
Introduction
Getting side effects from vaccines is common. Most are mild and go away in a few days, like soreness at the site or low-grade fevers, with severe allergic reactions only occurring in approximately 1 out of every million doses.
So why do side effects happen at all? It may actually be a good sign. These reactions are sometimes your body’s way of showing that it’s learning to protect you from future illness. That said, not everyone experiences side effects, and that’s okay too. Even without any side effects, your body can still build strong protection from the vaccine.
Learn more about what vaccine side effects are, why they happen, and how to tell the difference between a normal response and something more serious.
What to know about vaccine safety
Vaccines go through very careful testing before they’re approved for public use in the United States by the Food and Drug Administration (FDA). They’re studied in large groups of people, and monitored continuously for effectiveness and safety once they’re available to the public.
Most vaccine side effects, if they occur, appear within 6 weeks of administration, which is why studies follow participants for at least that long.
The Centers for Disease Control and Prevention also uses systems like Vaccine Adverse Event Reporting System (VAERS), Vaccine Safety Datalink (VSD), and the V-safe vaccine safety monitoring system to help track and monitor the safety of vaccines. Visit the CDC’s Vaccine Safety Page for more information.
How vaccines work:
Vaccines help train your immune system to recognize and fight off specific infections. They do this by introducing a piece of the virus or bacteria—like a protein or a weakened version of the germ—so your body can learn to defend itself. Some side effects are a result of this learning process.
Visit “What are vaccines and how do they help protect us?” to learn more about the different types of vaccines and how they work.
Vaccines help train your immune system to recognize and fight off specific threats.
What do mild vaccine side effects like soreness or fatigue usually indicate?
Correct
The immune system is learning to respond to a virus
Not quite
Correct answer: The immune system is learning to respond to a virus
Categories of vaccine side effects
Local reactions: At the injection site
When you receive a vaccine, it is common to have some reactions where the shot was given—like soreness, redness, or swelling. These are usually the first signs that your immune system is starting to respond. They are common side effects and usually go away on their own within a few days.
Systemic reactions: Affecting the whole body
Vaccines can also lead to effects across the body. These might show up as fatigue, mild headaches, or a low-grade fever, indicating that your immune system is actively responding. These common side effects typically develop shortly after vaccination, and go away within a couple of days.
Less common but not unusual
Less common side effects include vomiting, temporary body pain or stiffness. Although less frequent, these may require rest and drinking fluids. It's essential to follow these side effects closely and call your healthcare provider, especially if they continue or worsen. Additionally, some people may faint shortly after vaccination. For this reason, it is standard practice to receive a vaccine while seated or lying down and to remain under observation for at least 15 minutes afterward.
Rare side effects: Seek medical attention
While rare, some side effects are serious and need immediate medical care, such as severe allergic reactions like hives, general swelling, or difficulty breathing. These usually occur within minutes to hours after vaccination. That’s why it’s important to follow how you feel after vaccination and call your healthcare provider if something doesn’t feel right. If you develop a serious side effect such as severe shortness of breath or confusion, call 911.
Inflammation of the heart after COVID-19 vaccination is rare, and most patients respond well to medicine and rest and feel better quickly.
Other less common adverse effects can include high fever (over 105°F), disorders that affect the nervous system such as Guillain-Barré Syndrome (GBS), or a temporary reduction in the body’s cells that lead to clotting. These events are very uncommon and are closely reviewed by Food and Drug Administration and Centers for Disease Control and Prevention.
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As with any medicine, there is a very remote chance of a vaccine causing a severe allergic reaction, other serious injury, or death. In these rare case, severe problems are thoroughly investigated to learn about the role, if any, of the vaccine in causing the problem.
Special populations and considerations
While vaccines are safe for the general population, certain groups—like older adults, pregnant women, and those with long standing medical problems—may have different side effects or have special concerns when it comes to vaccination.
Older adults
Older adults may have fewer side effects due to an aging immune system, but vaccines are still highly effective in preventing serious illness.
People with long standing medical problems
Vaccines are especially important for people with underlying health problems (e.g., diabetes, asthma). Talk to your healthcare provider about any concerns, especially if you’re on medicines that weaken your immune system (e.g., cancer therapy, steroids, drugs to prevent rejection in persons with organ transplants such as the kidney).
Pregnancy
Vaccines like the flu shot, RSV, and Tdap are recommended during pregnancy to either help protect the mother, provide protection against certain infections in the newborn for their first months of life, or both. Always talk to the healthcare provider aiding you during your pregnancy about which vaccines are recommended during each stage of pregnancy. In addition to the vaccines listed above, other vaccines may be recommended in pregnancy if there is a special need for them (e.g., rabies).
Understanding side effects: Confidence through knowledge
Vaccine side effects can be uncomfortable, but they’re sometimes a sign that your body is doing exactly what it’s supposed to: building protection. Understanding what’s normal—and what’s not—can help you feel more confident and informed. If you ever have questions or concerns, don’t hesitate to reach out to your healthcare provider.
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
About Vaccines and Pregnancy. The Centers for Disease Control and Prevention. June 17, 2024. Accessed June 25, 2025. https://www.cdc.gov/vaccines-pregnancy/about/index.html
Autism and Vaccines. The Centers for Disease Control and Prevention. December 30, 2024. Accessed June 25, 2025. https://www.cdc.gov/vaccine-safety/about/autism.html
COVID-19 vaccines - what to expect. MedlinePlus. January 1, 2025. Accessed June 25, 2025. https://medlineplus.gov/ency/patientinstructions/000983.htm
Do Vaccines Cause Fibromyalgia or Chronic Fatigue Syndrome? Johns Hopkins Institute for Vaccine Safety. November 8, 2023. https://www.vaccinesafety.edu/do-vaccines-cause-fibromyalgia-or-chronic-fatigue-syndrome
Fainting and Vaccines. The Centers for Disease Control and Prevention. December 20, 2024. Accessed June 25, 2025. https://www.cdc.gov/vaccine-safety/about/fainting.html
Ginglen JG, Doyle MQ. Immunization. In: StatPearls. StatPearls Publishing; 2025.
Global immunization efforts have saved at least 154 million lives over the past 50 years. The World Health Organization. April 24, 2024. https://www.who.int/news/item/24-04-2024-global-immunization-efforts-have-saved-at-least-154-million-lives-over-the-past-50-years
Guidelines for Vaccinating Pregnant Women. The Centers for Disease Control and Prevention. July 12, 2024. Accessed July 21, 2025. https://www.cdc.gov/vaccines-pregnancy/hcp/vaccination-guidelines/index.html
Hause AM, Moro PL, Baggs J, et al. Early Safety Findings Among Persons Aged ≥60 Years Who Received a Respiratory Syncytial Virus Vaccine - United States, May 3, 2023-April 14, 2024 [published correction appears in MMWR Morb Mortal Wkly Rep. 2024 Jul 11;73(27):612. doi: 10.15585/mmwr.mm7327a4.]. MMWR Morb Mortal Wkly Rep. 2024;73(21):489-494. Published 2024 May 30. doi:10.15585/mmwr.mm7321a3
Hermann EA, Lee B, Balte PP, et al. Association of symptoms after covid-19 vaccination with anti–sars-cov-2 antibody response in the framingham heart study. JAMA Netw Open. 2022;5(10):e2237908.
How Vaccines are Developed and Approved for Use. The Centers for Disease Control and Prevention. August 10, 2024. Accessed June 25, 2025. https://www.cdc.gov/vaccines/basics/how-developed-approved.html
Ison MG. Vaccines for use in special populations: immunocompromised hosts. The Journal of Infectious Diseases. 2025;231(3):552-555.
Peikert A, Claggett BL, Kim K, et al. Association of post‐vaccination adverse reactions after influenza vaccine with mortality and cardiopulmonary outcomes in patients with high‐risk cardiovascular disease: the INVESTED trial. European J of Heart Fail. 2023;25(2):299-310.
Possible Side Effects from Vaccines. The Centers for Disease Control and Prevention. July 31, 2024. Accessed June 25, 2025. https://www.cdc.gov/vaccines/basics/possible-side-effects.html
Preventing and Managing Adverse Reactions. The Centers for Disease Control and Prevention. July 25, 2024. Accessed June 25, 2025. https://www.cdc.gov/vaccines/hcp/imz-best-practices/preventing-managing-adverse-reactions.html
Vaccines and immunization. The World Health Organization. 2025. Accessed June 25, 2025. https://www.who.int/health-topics/vaccines-and-immunization
Vaccine Side Effects. The United States Department of Health and Human Services. May 6, 2022. Accessed June 25, 2025. https://www.hhs.gov/immunization/basics/safety/side-effects/index.html
Vaccine side effects signal their effectiveness. The National Heart, Lung, and Blood Institute. November 30, 2022. Accessed June 25, 2025. https://www.nhlbi.nih.gov/news/2022/vaccine-side-effects-signal-their-effectiveness