A fever after vaccination is one of the most common reactions in babies, and in the vast majority of cases, it’s completely normal. Roughly 1 in 4 infants will develop a mild fever after routine shots. The fever is a sign your baby’s immune system is responding to the vaccine and building protection. Here’s what you need to know and what steps to take.
Why Vaccines Cause Fevers
Vaccines work by introducing harmless components of a germ to train your baby’s immune system. When that immune response kicks into gear strongly enough, one of the side effects is a rise in body temperature. This isn’t a sign something went wrong. A fever actually helps the immune process in two ways: it makes the body a less hospitable environment for germs, and it activates signaling chemicals that guide the immune response more effectively.
The likelihood of fever depends on which vaccines your baby received. The DTaP vaccine (diphtheria, tetanus, pertussis) triggers a fever in about 25% of children. Combination vaccines like Pediarix cause fever in 28 to 39 out of 100 infants. The pneumococcal vaccine causes mild fever in about 15% of babies, while hepatitis B causes it in only about 3%. If your baby got multiple vaccines at the same visit, the chance of a fever goes up simply because the immune system is responding to more than one thing at once.
When the Fever Starts and How Long It Lasts
For most routine infant vaccines, fever appears within the first 24 hours after the shot and resolves within 24 to 48 hours. The DTaP vaccine, for example, typically causes a fever that lasts one to two days. The pneumococcal vaccine follows a similar pattern of one to two days.
There are a few exceptions worth knowing about. The MMR vaccine (measles, mumps, rubella) can cause a delayed fever that shows up 7 to 12 days after the shot and lasts two to three days. The chickenpox vaccine can cause a mild fever 14 to 28 days later. So if your baby develops a fever a week or two after those particular vaccines, that’s still within the expected range.
How to Help Your Baby Feel Better
Most post-vaccination fevers are mild, staying below 102°F (39°C), and your baby may not need any medication at all. Focus first on comfort measures: dress your baby in light clothing, offer extra fluids (breast milk, formula, or water if they’re old enough), and keep the room at a comfortable temperature. A lukewarm sponge bath can help if your baby seems uncomfortable.
If your baby is clearly fussy or in discomfort, acetaminophen (Tylenol) is an option. The liquid form for infants comes in a standard concentration of 160 mg per 5 mL. Dosing is based on your baby’s weight, not age, and you can give it every four hours as needed. For babies under 2 years old, check with your pediatrician for the correct dose before giving it. Ibuprofen (Motrin, Advil) is not safe for babies under 6 months old. For babies 6 months and older, ibuprofen can be given every 6 to 8 hours based on weight.
One important note: don’t give fever-reducing medication before the vaccination appointment as a preventive measure. Some evidence suggests that giving acetaminophen before the shot may slightly reduce the vaccine’s effectiveness. The American Academy of Pediatrics notes that some pediatricians no longer recommend this practice. Wait until after the shot, and only use it if your baby actually seems uncomfortable.
Temperature Thresholds That Matter by Age
The rules for when a fever needs medical attention vary significantly based on how old your baby is, even when you know the fever is likely from a vaccine.
- Under 3 months: Call your pediatrician for any fever, regardless of the number. A rectal temperature of 100.4°F (38°C) or higher in this age group always warrants a call, even after vaccination.
- 3 to 6 months: Call if the temperature reaches 100.4°F (38°C) or higher, or if your baby has a lower temperature but seems unusually sick or lethargic.
- 6 to 24 months: Call if the fever is above 100.4°F (38°C) and lasts more than one day.
- Any age: Call if the fever persists beyond three days.
A rectal thermometer gives the most accurate reading for infants. If you’re unsure whether your baby’s temperature is high enough to be concerned about, it’s always reasonable to call your pediatrician’s office. Most have nurse lines specifically for these kinds of questions.
Signs That Need Immediate Attention
While a mild fever and some fussiness are expected, certain symptoms after vaccination are not normal and require urgent medical care. These are rare, but you should know what to watch for.
Signs of a serious allergic reaction (anaphylaxis) typically appear within minutes to hours after the shot. In babies, these can look like:
- Hives, facial flushing, or swelling of the face, lips, or tongue
- Difficulty breathing, coughing, or wheezing
- Excessive drooling
- Persistent vomiting
- Sudden sleepiness or limpness, or crying that won’t stop
- Loss of bladder or bowel control
If you notice any combination of these symptoms, call emergency services immediately. Severe allergic reactions to vaccines are very uncommon, but they require fast treatment. This is one reason vaccination sites typically ask you to wait 15 minutes after the shot before leaving.
What’s Normal Fussiness vs. Something More
It can be hard to tell the difference between a baby who’s just uncomfortable from the shot and one who needs medical attention. A baby with a normal vaccine reaction may be crankier than usual, sleep more or less than normal, eat a bit less, and have a warm forehead. The injection site may be red, swollen, or tender to the touch. All of this is expected and resolves on its own.
What tips the balance toward concern is a combination of high fever and behavioral changes that go beyond garden-variety fussiness. A baby who is unusually limp, unresponsive, inconsolable for hours on end, or refusing all fluids is worth a call to your pediatrician. Trust your instincts on this. You know your baby’s normal range of behavior better than anyone, and pediatricians expect these calls after vaccine visits.