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.
By Age
What to expect by age
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.
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.
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.
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
Related Medical Concerns
My Baby's Head Shape Looks Abnormal
Many babies develop temporary head shape irregularities that are completely normal. A cone-shaped head from vaginal delivery reshapes within days. Mild positional flattening (plagiocephaly) from sleeping on the back is very common and usually improves with repositioning and tummy time. However, head shape changes involving ridges, a persistently bulging fontanelle, or rapid head growth changes should be evaluated to rule out craniosynostosis.
I'm Worried About Lazy Eye (Amblyopia)
Amblyopia (lazy eye) is the most common cause of vision loss in children, affecting about 2-3% of kids. It occurs when one eye develops weaker vision because the brain favors the other eye. The tricky part is that amblyopia often has no obvious outward signs - the eye usually looks normal. Early detection through routine vision screening is critical because treatment is most effective in the first few years of life.
Anaphylaxis Signs in Baby
Anaphylaxis is a severe, potentially life-threatening allergic reaction that affects multiple body systems. In babies, it can be caused by food (most commonly), insect stings, or medications. Signs include widespread hives, facial or throat swelling, difficulty breathing, persistent vomiting, and becoming limp or unresponsive. Anaphylaxis is a medical emergency. If you suspect anaphylaxis, use an epinephrine auto-injector if available and call 911 immediately. Early recognition and rapid treatment lead to excellent outcomes in the vast majority of cases.
My Baby Has Unequal Pupils
Slight differences in pupil size (anisocoria) can be normal and affect up to 20% of people, including babies. However, if the difference is large, came on suddenly, or is accompanied by other symptoms like drooping eyelid, vision changes, or neurological symptoms, it needs immediate medical evaluation to rule out serious causes.
My Baby Stops Breathing Briefly (Apnea)
Brief pauses in breathing lasting under 10 seconds are very common in newborns and are called periodic breathing. This is a normal pattern where the baby breathes rapidly, then pauses briefly, then resumes. However, true apnea (pauses lasting 20 seconds or longer, or shorter pauses accompanied by color changes or heart rate drops) is a medical concern that should be evaluated promptly.
Baby Allergic Reaction to Food
Food allergic reactions in babies range from mild (hives, rash around the mouth, minor vomiting) to severe (difficulty breathing, widespread swelling, multiple body systems affected). Most reactions are mild and appear within minutes to 2 hours after eating the food. The most common food allergens in babies are milk, eggs, peanuts, tree nuts, soy, wheat, fish, and shellfish. Current AAP guidelines recommend introducing allergenic foods around 6 months, as early introduction can actually help prevent allergies in many cases.