Post-Vaccine Fever
The short answer
A low-grade fever after vaccination is one of the most common and expected side effects. It means your baby's immune system is responding to the vaccine and building protection - which is exactly what it is supposed to do. Most post-vaccine fevers are mild (under 102 degrees F), begin within 6-24 hours after the shot, and resolve within 1-2 days without any treatment needed.
Parents everywhere have the same worry. You are doing the right thing by looking into it.
By Age
What to expect by age
0-2 months
The first vaccines (Hepatitis B) are given at birth and 1 month. Fever after these vaccines is uncommon but possible. Because any fever in a baby under 2 months old needs medical attention regardless of cause, contact your pediatrician if your newborn has a rectal temperature of 100.4 degrees F (38 degrees C) or higher after vaccination.
2-6 months
The 2-month, 4-month, and 6-month vaccine visits involve multiple shots and are the most common times for post-vaccine fever. A temperature up to 102 degrees F (38.9 degrees C) within 24-48 hours is typical. You can offer infant acetaminophen (Tylenol) if your baby is uncomfortable, but do not give it preventively before the vaccine as some research suggests it may slightly reduce the immune response.
6-18 months
Fever may occur after the 12-month and 15-month vaccines, which include MMR and varicella. The MMR vaccine can cause a delayed fever appearing 7-12 days after the shot, which can surprise parents who are not expecting it. This delayed fever is caused by the body's response to the live vaccine components and is normal. It typically lasts 1-2 days.
18 months - 5 years
Booster shots can still cause brief fevers, though children at this age often have milder reactions than younger babies. The DTaP booster at 15-18 months and 4-6 years is most commonly associated with fever and local soreness. If your child has a fever that begins more than 48 hours after a non-live vaccine or lasts more than 3 days, it is likely from a coincidental illness rather than the vaccine.
What Should You Do?
When to take action
- Your baby has a fever under 102 degrees F (38.9 degrees C) that begins within 24 hours of vaccination
- The fever resolves within 1-3 days without worsening
- Your baby is fussy but still feeding, making wet diapers, and able to be comforted
- Mild redness, swelling, or a small hard lump at the injection site
- A fever appearing 7-12 days after the MMR vaccine, lasting 1-2 days
- The fever is above 102 degrees F (38.9 degrees C) or lasts more than 3 days after a non-live vaccine
- Your baby has a rash, excessive crying for more than 3 hours, or seems more ill than you would expect from a simple vaccine reaction
- Any fever of 100.4 degrees F (38 degrees C) or higher in a baby under 2 months old, regardless of vaccination
- Your baby has a fever above 104 degrees F (40 degrees C), is extremely lethargic, inconsolable, has difficulty breathing, or shows signs of an allergic reaction such as hives, swelling of the face or throat, or wheezing
Sources
Related Resources
Trust your instincts. If something feels wrong, reach out to your pediatrician.
Worrying about your baby means you care. That is a good thing.
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