Vaccine Side Effects: Normal vs. When to Worry

Most babies experience mild, temporary side effects after vaccination. Here is what is normal, what to watch for, and when to call your pediatrician.

Content reviewed against published CDC, AAP guidelines

Last reviewed: July 2026 · Editorial policy

Last reviewed:

Key takeaways

  • Mild side effects (fever, fussiness, soreness) are normal signs that the immune system is building protection.
  • Most side effects resolve within 24-48 hours without treatment.
  • Serious reactions are extremely rare — about 1 in a million doses for anaphylaxis.
  • You do not need to give Tylenol before vaccines, but you can give it afterward if needed for comfort.
  • Mild reactions are NOT a reason to delay or skip future vaccine doses.

Common Side Effects (Normal)

These reactions are common, expected, and a sign that the vaccine is working. They typically appear within hours of vaccination and resolve in 1-2 days:

Pain/redness at injection site

Affects up to 50% of babies. Apply a cool washcloth for comfort.

Low-grade fever

Under 101°F (38.3°C). Normal immune response. Resolves in 1-2 days.

Fussiness/irritability

Baby may be cranky for 24-48 hours. Extra cuddles and feeding help.

Mild swelling

Small lump or swelling at injection site. Resolves on its own.

Decreased appetite

May eat less for 1-2 days. Keep offering feeds.

Sleepiness

More tired than usual for a day. Let baby rest.

Less Common but Still Normal

  • Mild rash (after MMR or varicella) — A few spots may appear 7-14 days after vaccination. Not contagious in most cases.
  • Temporary joint stiffness (after MMR in older children) — Resolves without treatment.
  • Mild diarrhea (after rotavirus) — May last a few days. Keep baby hydrated.
  • Small hard lump at injection site — May persist for weeks. Harmless and resolves on its own.

When to Call Your Pediatrician

Contact your doctor if your baby experiences:

  • Fever above 104°F (40°C)
  • Crying or fussiness that lasts more than 3 hours and cannot be consoled
  • Redness or swelling at the injection site that increases after 24 hours
  • Fever that lasts more than 3 days
  • Unusual lethargy or difficulty waking
  • Any symptom that concerns you as a parent

Rare: Emergency Signs (Call 911)

These are extremely rare (about 1 in a million doses) but require immediate emergency care:

  • Difficulty breathing or wheezing
  • Swelling of the face, lips, or throat
  • Hives spreading over the body
  • Sudden paleness or limpness
  • Seizure or convulsion
  • High-pitched inconsolable screaming for hours

Note: Severe allergic reactions (anaphylaxis) almost always occur within 15-30 minutes of vaccination. This is why your provider asks you to wait 15 minutes after shots.

Comfort Measures After Shots

  • Breastfeed or offer a bottle during or immediately after — sucking provides natural pain relief
  • Hold and cuddle your baby. Skin-to-skin contact helps calm them
  • Apply a cool, damp washcloth to the injection site for soreness
  • Move or gently massage the vaccinated leg/arm to reduce stiffness
  • Dress baby in loose clothing to avoid irritating the injection site
  • Offer infant acetaminophen (Tylenol) if recommended by your pediatrician for fever or pain
  • Avoid ibuprofen (Motrin/Advil) in babies under 6 months

Related Vaccine Topics

After Vaccination

Full guide to post-vaccination care

Vaccine Safety

How vaccines are tested and monitored

Frequently asked questions

How long do vaccine side effects last in babies?
Most common side effects — low-grade fever, fussiness, soreness at the injection site, and mild swelling — resolve within 24-48 hours. Rotavirus vaccine may cause mild temporary diarrhea for a few days. If symptoms persist beyond 3 days or worsen, contact your pediatrician.
Can I give my baby Tylenol before vaccines to prevent side effects?
The AAP does not routinely recommend giving acetaminophen (Tylenol) before vaccination, as some studies suggest it may slightly reduce the immune response. However, you can give an appropriate dose after vaccination if your baby develops fever or pain. Always use the correct dosage based on your baby's weight and check with your pediatrician.
Is a fever after vaccination normal?
Yes. A low-grade fever (under 101°F / 38.3°C) is a normal sign that your baby's immune system is responding to the vaccine. It typically appears within 24 hours and resolves within 1-2 days. Call your doctor if fever exceeds 105°F (40.5°C), lasts more than 3 days, or your baby is under 3 months with any fever.
What are signs of a serious vaccine reaction?
Serious reactions are extremely rare (about 1 in a million doses for anaphylaxis). Call 911 or go to the ER immediately if your baby has: difficulty breathing, swelling of the face or throat, hives all over the body, rapid heartbeat, dizziness or weakness, or a high-pitched unusual cry lasting more than 3 hours. These reactions almost always occur within 15-30 minutes of vaccination.
Should I skip vaccines if my baby had a mild reaction last time?
No. Mild reactions like low-grade fever and fussiness are normal and not a reason to skip future doses. Only true contraindications — such as a severe allergic reaction (anaphylaxis) to a previous dose or vaccine component — warrant changes. Talk to your pediatrician about any concerns.

Bottom line

Mild side effects after vaccination are normal and temporary — they mean the immune system is doing its job. Serious reactions are extremely rare. If anything worries you, call your pediatrician. Never skip future doses because of a mild reaction.

It is hard to watch your baby be uncomfortable, even briefly. You are protecting them from much more serious illnesses. That discomfort passes quickly.

Our commitment to accuracy

Every page is reviewed against published guidelines from authoritative medical organizations including the CDC, WHO, AAP, ACOG, ASHA, and NIH.

2,705 evidence-based guides6 authoritative medical sources

Editorial PolicyMedical Review ProcessSources & References