Common Vaccine Side Effects: What Is Normal and What to Watch For

What are normal local reactions after a vaccine?

The most common side effects are at the injection site itself: soreness, redness, warmth, or mild swelling 1Ref 1Wodi AP, Issa AN, Moser CA, Cineas S (2025).Advisory Committee on Immunization Practices Recommended Immunization Schedule for Adults Aged 19 Years or Older — United States, 2025.General characterization of vaccine side effects, local vs. systemic reactions, anaphylaxis as a rare event, and the clinical approach to vaccine safety monitoring. These typically appear within hours of the shot and peak in the first day or two. Applying a cool cloth to the area, gently moving the arm, and over-the-counter pain relievers (guided by a clinician or pharmacist for appropriate type and dose) can help with discomfort.

A small firm bump at the site may persist for a week or two and is generally not a concern.

What whole-body reactions are normal after a vaccine?

Some people experience systemic (whole-body) reactions after vaccination, including low-grade fever, fatigue, headache, muscle aches, chills, or nausea 1Ref 1Wodi AP, Issa AN, Moser CA, Cineas S (2025).Advisory Committee on Immunization Practices Recommended Immunization Schedule for Adults Aged 19 Years or Older — United States, 2025.General characterization of vaccine side effects, local vs. systemic reactions, anaphylaxis as a rare event, and the clinical approach to vaccine safety monitoring. These effects are more commonly reported with certain vaccines — particularly those that produce a strong immune response, such as COVID-19 mRNA vaccines and Shingrix (shingles vaccine). They typically last one to three days.

Rest, fluids, and appropriate over-the-counter comfort measures help most people through this period. Ask a pharmacist or clinician before taking any pain reliever if you have questions about what is safe for you.

What should I expect after my child's routine shots?

Infants and young children often receive multiple vaccines in one visit. Crying, fussiness, and drowsiness are very common in the hours after shots 2Ref 2Issa AN, Wodi AP, Moser CA, Cineas S (2025).Advisory Committee on Immunization Practices Recommended Immunization Schedule for Children and Adolescents Aged 18 Years or Younger — United States, 2025.Fussiness, low-grade fever, and local reactions are common and expected in infants after routine multi-vaccine visits; guidance on post-vaccination observation. Low fever and soreness at the injection site are also typical. Most reactions resolve within 24 to 48 hours.

Contact your child's doctor if the fever is high, the child seems unusually inconsolable, or symptoms last beyond a couple of days — they can help you decide whether a visit is needed.

What less common reactions are worth knowing about?

A small number of people develop a more pronounced local reaction — significant swelling of the entire upper arm, for example — or a higher fever 1Ref 1Wodi AP, Issa AN, Moser CA, Cineas S (2025).Advisory Committee on Immunization Practices Recommended Immunization Schedule for Adults Aged 19 Years or Older — United States, 2025.General characterization of vaccine side effects, local vs. systemic reactions, anaphylaxis as a rare event, and the clinical approach to vaccine safety monitoring. Fainting (vasovagal syncope) can occur shortly after any injection, which is why providers ask you to wait 15 minutes after vaccination 3Ref 3Centers for Disease Control and Prevention (2024).Chapter 2: General Best Practice Guidance for Immunization (Epidemiology and Prevention of Vaccine-Preventable Diseases, The Pink Book).Vasovagal syncope after vaccination occurs most often within 15 minutes; 80% of reported episodes occur within this window; providers should observe vaccinated patients seated or lying down for 15 minutes to reduce injury risk. Among all age groups, about 80% of reported syncope episodes occur within that 15-minute window.

Swollen lymph nodes near the injection site, particularly after mRNA COVID-19 vaccines, have been reported and generally resolve on their own. Let your clinician know if you experience anything beyond mild local or systemic effects.

Some vaccines produce more robust side effects than others. This does not mean one is less safe than another — it reflects differences in how they stimulate the immune system 1Ref 1Wodi AP, Issa AN, Moser CA, Cineas S (2025).Advisory Committee on Immunization Practices Recommended Immunization Schedule for Adults Aged 19 Years or Older — United States, 2025.General characterization of vaccine side effects, local vs. systemic reactions, anaphylaxis as a rare event, and the clinical approach to vaccine safety monitoring.

When is a reaction after a vaccine actually a problem?

The distinction between expected reactions and concerning ones comes down to a few key signals:

• Improving vs. worsening: Normal reactions improve within a few days. An injection site that is redder, warmer, and more painful after 48 hours — especially with spreading redness — suggests a possible infection and warrants evaluation.
• Degree of fever: Low-grade fever is common. High fever (above 104°F / 40°C in adults, or any fever in a newborn under 2 months) is not expected and should prompt a call to a clinician.
• Respiratory or circulatory symptoms: Difficulty breathing, hives, or swelling of the face or throat within minutes to hours of a vaccine are signs of anaphylaxis — a rare but serious allergic reaction requiring emergency treatment 1Ref 1Wodi AP, Issa AN, Moser CA, Cineas S (2025).Advisory Committee on Immunization Practices Recommended Immunization Schedule for Adults Aged 19 Years or Older — United States, 2025.General characterization of vaccine side effects, local vs. systemic reactions, anaphylaxis as a rare event, and the clinical approach to vaccine safety monitoring.