Post-Vaccine Fever
Medically reviewed by Dr. Michael Okonkwo, MD, FAAP · Board-Certified Neonatologist
Content reviewed against published CDC, AAP, NIH guidelines
Last reviewed:
If your baby has been diagnosed with or you suspect post-vaccine fever, here is what the evidence says.
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.
Key takeaways
- 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.
- Usually normal when: Your baby has a fever under 102 degrees F (38.9 degrees C) that begins within 24 hours of vaccination
- Call your doctor if: Any fever of 100.4 degrees F (38 degrees C) or higher in a baby under 2 months old, regardless of vaccination
- Varies by age — see the age-by-age breakdown below
“Fever itself is not an illness — rather, it is a sign or symptom that the body is fighting an infection. Fever stimulates certain defenses, such as the white blood cells, which attack and destroy invading bacteria.”
Parents everywhere have the same worry. You are doing the right thing by looking into it.
What Parents Should Know
According to CDC, AAP, NIH guidelines, 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. At 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. It is generally considered normal when your baby has a fever under 102 degrees F (38.9 degrees C) that begins within 24 hours of vaccination. However, you should contact your pediatrician promptly if any fever of 100.4 degrees F (38 degrees C) or higher in a baby under 2 months old, regardless of vaccination.
Normal vs. Concerning
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 to Tell Your Pediatrician
- Describe when you first noticed post-vaccine fever and how it has changed over time.
- Note your baby's current age and which age-specific patterns you are seeing.
- Mention if the fever is above 102 degrees F (38.9 degrees C) or lasts more than 3 days after a non-live vaccine.
- Mention if 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.
- Bring a list of any questions or observations you want to discuss at the appointment.
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
What You Can Do at Home
- Keep track of when you notice post-vaccine fever — noting the time of day, duration, and any triggers can help your pediatrician.
- Remember that your baby has a fever under 102 degrees F (38.9 degrees C) that begins within 24 hours of vaccination — this is generally within the range of normal.
- At 0-2 months, focus on observation rather than intervention unless your pediatrician advises otherwise.
- Follow any care instructions from your pediatrician. Keep a written log of symptoms to bring to appointments.
- While monitoring at home, seek immediate care if any fever of 100.4 degrees F (38 degrees C) or higher in a baby under 2 months old, regardless of vaccination.
Related Resources
Fever Decision Tree
Step-by-step guidance for evaluating your baby's fever by age and temperature.
Vaccine Schedule Tracker
Track vaccinations, see what's due, and understand each vaccine.
Month-by-Month Development
Detailed monthly development guides from birth through 24 months.
When to Call the Doctor
General guide on when to call the pediatrician, visit urgent care, or go to the ER.
Frequently asked questions
Is post-vaccine fever normal?
When should I call the doctor about post-vaccine fever?
When is post-vaccine fever normal?
What causes post-vaccine fever?
What should I mention to my pediatrician about post-vaccine fever?
Is post-vaccine fever normal at 0-2 months?
Is post-vaccine fever normal at 2-6 months?
Should I go to the ER for post-vaccine fever?
Does post-vaccine fever go away on its own?
References
Doctor Visit Checklist
Bring this checklist to your next pediatrician visit to discuss Post-Vaccine Fever.
Things to mention
- Describe when you first noticed post-vaccine fever and how it has changed over time.
- Note your baby's current age and which age-specific patterns you are seeing.
- Mention if the fever is above 102 degrees F (38.9 degrees C) or lasts more than 3 days after a non-live vaccine.
- Mention if 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.
- Bring a list of any questions or observations you want to discuss at the appointment.
Observations to share
- 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
Urgent signs to report immediately
- 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
My notes
From ismybabyalright.com — free, evidence-based baby health guides
All content follows our editorial policy and is reviewed against published clinical guidelines.
2,705 evidence-based guides6 authoritative medical sources5 medical advisory board members
Related Resources
Bottom line
Most cases of post-vaccine fever are normal. Talk to your pediatrician if any fever of 100.4 degrees f (38 degrees c) or higher in a baby under 2 months old, regardless of vaccination.
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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