Common Reactions

While everyone's attention tends to focus on the serious and long-lasting side effects of vaccines, you must not overlook the relatively minor reactions that are commonly associated with vaccination. Even though these reactions are mild and tend to go away on their own, they can nevertheless be the source of parental stress and anxiety. This section will explore these reactions and discuss ways to ameliorate them.


Most parents know that children can develop a fever after routine vaccination, but you may not know just how often a fever can occur and when to call the doctor about a fever. Not all vaccines cause fever after administration, but most can. Even among the vaccines that may trigger a fever, only about 10–20 percent of children will develop a fever after these vaccines. So even though fever is a common postvaccination reaction, it does not happen more than half of the time. It is very likely that your child may be completely fine after receiving her vaccination.


The following vaccines only cause fever in rare circumstances: the HPV vaccine, the hepatitis A vaccine, the meningococcal vaccine, and the polio vaccine. These vaccines differ from most other vaccines because they do not trigger a fever in most cases.

Fever is your body's alarm system. When the immune system goes into high-alert mode, it sends a signal to the body, and the part of your brain that controls body temperature elevates the set temperature for the entire body and you develop a fever.

In children, fevers usually happen because there is an infection. In case of vaccination, there is no “real” infection going on, but the immune system can go into a high-alert mode because it may perceive that there is a germ invasion going on. Think of the fever triggered by vaccination as the body's defense system having a mock drill in preparation for a germ invasion. While there is no threat, the defenses are up and the body learns how to fight off a real infection if it were to occur in the future.

The technical definition of a fever is when your child's temperature rises above 100.3°F (38° Celsius). While the exact body temperature is important when determining the presence of a fever, how the temperature is taken and the circumstances when the temperature is taken are equally important.

There are several types of thermometers and several ways you can take the temperature of your child. If your baby is less than three months old, the best and most reliable way of taking the temperature is through a digital rectal thermometer. However, this method of measuring body temperature can be slightly uncomfortable for the baby, and some parents may feel uneasy with the procedure.


The ear thermometer, or the tympanic thermometer, scans the body temperature by reading the infrared radiation from the body. While it is quick and easy to take the temperature, the temperature reading from this type of thermometer is inaccurate and unreliable, especially if your child is less than six months of age.

The next best way to measure the temperature is in the mouth, but this is only feasible for older children who will not try to eat the thermometer when you put it inside the mouth. Inserting the thermometer under the armpit (also called the axillary temperature) is a good compromise between ease of taking the temperature and accuracy. While the body temperature measured in the armpit tends to be somewhat lower than the actual rectal temperature, it is close enough for the parents to get an idea of the body temperature. Even though many thermometer instructions recommend adding a degree or two to an axillary temperature, doing so may overestimate the actual temperature and cause a false alarm for fever.

Finally, you need to take your child's surroundings into consideration when interpreting a high temperature. If your child is bundled up in several thick layers of wooly blanket on a hot summer afternoon, the body temperature can rise significantly. This doesn't necessarily mean that your child has a fever. If the recorded temperature is high and your baby may be too hot from overbundling, unwrap the baby for ten minutes and recheck the temperature. If the temperature continues to be high, then a real fever may be present.

While the fever triggered by vaccination is not dangerous, it can still be alarming to parents and uncomfortable for the child. A vaccine-induced fever is usually very short lasting. Such fever rarely lasts for more than two days, and three days of fever after immunization is quite unexpected.


If your child has a fever for more than two days after vaccination, you should take him to the doctor's office and have him checked out for other causes of the fever. Your doctor may not elect to do any blood tests or x-rays at that time, depending on your child's physical examination in the office.

What if the fever is very high after vaccination? Is that a cause for concern? Not necessarily. First of all, a high fever is when the body's temperature rises above 102°F. Two to five percent of children may develop a fever that high after vaccination, so it would not be too unusual. However, if the temperature goes higher than 104°F after vaccination, most doctors would consider doing some evaluation (such as blood tests) to ensure that there is nothing else that could be causing the high fever. Whenever you are uncertain about any vaccine reaction, give your doctor a call.

Since most children will not develop a fever after vaccination, routine administration of over-the-counter fever reducers (acetaminophen, ibuprofen) beforehand is generally not recommended. Giving every child medication to prevent a minority with fever is unnecessary at best and possibly harmful at times. Consult your doctor about the dosage of fever reducer if you plan to use these medications.

Besides using fever-reducing medications, you may also utilize other cooling measures for postvaccination fever. You can increase the fluid intake for your child, and you can soak a towel with room-temperature water and drape the wet towel over your child's bare shoulder. Remember, fever after vaccination is not dangerous. Measures to reduce the temperature are primarily to make your child more comfortable and reduce your own anxiety.


If you elect to use a fever reducer to treat your child's fever, use acetaminophen (Tylenol). You may use ibuprofen (Motrin) if your child is older than six months of age. Avoid using aspirin for any child under all circumstances because aspirin can cause a fatal reaction in children called Reye's syndrome.

Whether the fever is triggered by the vaccine or caused by a real infection, an elevated body temperature itself is never dangerous for your child. There is a popular belief that if the temperature gets too high it can cause permanent brain damage. This is never the case. No matter how high the fever goes, it will not cause harm to your child. The only exception to this rule is if your child's temperature is high because the surrounding environment does not allow the body to dissipate heat. If the body gets trapped in a very hot environment (hot tub, the desert) and cannot lower the temperature by sweating, the resulting high body temperature can lead to heat stroke, which can be life threatening. Generally speaking, body temperature never exceeds more than 106°F, unless the outside environment is the reason for the elevated temperature.


The skin where the injection was given can sometimes swell up after vaccination. This does not happen with the majority of injections, but it can occur up to 20 percent of the time with some vaccines. Even though it is relatively common, the good news is that the degree of swelling is not usually related to the amount of pain. It is quite common for children to have an extremely large swelling but have absolutely no pain in the swollen area.

The best thing to do for the swelling is apply a cool washcloth over the swollen area. You can also use ibuprofen to reduce the swelling, but avoid using aspirin in children. A heating pad is generally not recommended because it tends to worsen the swelling.


If the swelling covers a large area of the body or if the swelling is accompanied with significant pain or tenderness, contact the doctor immediately. Swelling after vaccination is usually painless or minimally painful. A very painful reaction is likely the result of poor vaccination technique.


This can vary anywhere from a mild fussiness to full-blown nonstop screaming for hours on end. Obviously, the difference between the degrees of severity of the reaction matters a great deal. Fortunately, the inconsolable crying that lasts more than three hours is an uncommon reaction that is only associated with the DTaP vaccine. Most children do not cry excessively after injections, and the crying rarely lasts for more than ten minutes. If the crying continues for more than an hour, call your doctor.

For mild, short-lasting irritability, you can calm your child by cradling her or rocking her in your arms. If your child wants to sleep, it is perfectly safe for her to take an early nap. Even though acetaminophen (Tylenol) has not been shown to be effective in reducing this irritability, it's a relatively safe remedy and you can administer it if you wish.


Your baby may become sleepier after getting vaccinated. He may take a longer nap than usual or actually sleep through the night following immunization. This reaction is not dangerous, and most parents actually welcome the rare respite after not being able to sleep eight hours straight for months.

This reaction is harmless and does not require any treatment. Your baby will usually return to his regular sleeping routine a day or two after the immunization. Enjoy the long naps while you can.

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