Most vaccine side effects in babies are mild and short-lived. Fussiness, low-grade fever, and soreness at the injection site are the most common reactions, and they typically resolve within one to three days. Serious reactions are exceptionally rare, occurring in fewer than 1 in 100,000 doses for the most concerning outcomes.
Why Vaccines Cause Side Effects
Vaccines work by triggering your baby’s immune system to build defenses against a specific disease. That immune activation is what causes the familiar side effects. The body sends immune cells to the injection site, producing localized warmth, redness, and swelling. Sometimes the response is widespread enough to cause a mild fever or general fussiness. These reactions sit on the normal spectrum of immune activity and are actually a sign that the body is responding as expected.
Common Reactions After Shots
The side effects parents notice most often fall into a few predictable categories. At the injection site, you may see redness, swelling, or a small hard lump that can last a few days. Your baby may cry more than usual and seem irritable, especially in the first 24 hours. A low-grade fever (under 101°F) is common and usually appears within a day of the shot.
Some babies are sleepier than normal or lose interest in feeding for a short time. Others become unusually clingy. These reactions are most noticeable after combination visits where multiple vaccines are given at once, simply because the immune system is responding to several antigens at the same time. Nearly all of these symptoms fade within 48 to 72 hours without any treatment beyond comfort measures.
The MMR Vaccine Has a Delayed Timeline
One vaccine that catches parents off guard is MMR (measles, mumps, and rubella), given at 12 months. Unlike most vaccines, its side effects show up much later. Some children develop a mild, measles-like rash about 7 to 12 days after the shot. A fever above 103°F can appear 5 to 12 days afterward. Because of this delay, parents sometimes don’t connect the symptoms to the vaccine and worry their child is sick with something new. The rash and fever from the MMR vaccine are not contagious and resolve on their own.
Rotavirus Vaccine and Digestive Symptoms
The rotavirus vaccine is given by mouth rather than injection, so its side effects look different. Mild, temporary diarrhea or vomiting can occur after a dose, along with general irritability. These digestive symptoms are usually brief.
There is one rare but serious risk specific to this vaccine: intussusception, a type of bowel obstruction where one segment of the intestine slides into another. The risk is estimated at roughly 1 case per 20,000 to 100,000 vaccinated infants, and it occurs most often within a week of the first or second dose. Signs include severe crying with legs pulled up, vomiting, bloody stool, or a lump in the abdomen. This is a medical emergency, but the overall risk remains very small compared to the danger of rotavirus disease itself.
Rare but Serious Reactions
Febrile seizures are the serious reaction parents fear most. These are convulsions triggered by a rapid rise in body temperature, not by the vaccine ingredients themselves. When certain vaccines are given together (particularly the flu shot alongside pneumococcal or DTaP vaccines), the risk reaches at most 30 febrile seizures per 100,000 children vaccinated. Febrile seizures are frightening to witness but almost always end on their own within a few minutes and do not cause lasting harm or brain damage.
Non-stop crying lasting 3 hours or more, or a fever above 105°F, can occur after DTaP vaccination but is uncommon. These warrant a call to your pediatrician.
Severe allergic reactions (anaphylaxis) are the rarest outcome. A large study published by the American Academy of Pediatrics found just 5 cases of potentially vaccine-associated anaphylaxis out of more than 7.6 million doses, a rate of roughly 0.65 cases per million. None of those cases resulted in death. Anaphylaxis almost always happens within 15 to 30 minutes of the shot, which is why your pediatrician’s office asks you to wait before leaving.
How Vaccine Safety Is Tracked
Vaccine safety monitoring doesn’t stop once a vaccine is approved. The CDC’s Vaccine Safety Datalink pulls electronic health records from multiple large healthcare systems across the country to look for patterns in near-real time. Every week, researchers compare the rate of specific health problems in recently vaccinated children against a comparison group. If the vaccinated group shows a higher rate beyond a set threshold, follow-up investigations begin immediately. This system also links records between pregnant women and their infants, allowing researchers to study outcomes across age groups and catch problems that might emerge only in very large populations.
A separate system, VAERS, accepts reports from anyone, including parents, and serves as an early warning signal. Together, these systems create a surveillance network that would detect a new or unexpected pattern quickly.
Managing Your Baby’s Discomfort
For injection-site soreness, a cool, damp cloth placed over the area can reduce swelling and ease pain. Gentle movement of the arm or leg where the shot was given also helps. Dress your baby in light clothing if they develop a fever, and offer extra fluids through breastfeeding or bottle-feeding.
A non-aspirin pain reliever can help with both fever and fussiness, but check with your pediatrician on the right dose for your baby’s weight before giving anything. Never give aspirin to an infant or young child. Extra cuddles and skin-to-skin contact go a long way during the first day or two.
Signs That Need Prompt Attention
Most post-vaccine symptoms are harmless, but a few warrant a quick call or visit. Contact your pediatrician if your baby has a fever above 105°F, cries inconsolably for more than 3 hours, seems unusually limp or unresponsive, or develops hives or facial swelling. After rotavirus vaccine specifically, watch for signs of intussusception: severe abdominal pain (often with knees drawn to chest), repeated vomiting, or bloody or jelly-like stool. These symptoms in the week following vaccination need immediate evaluation.