Vaccine Side Effects: What Parents Need to Know

Sometimes the "ouch" of a vaccine doesn't end right after the shot is given. Soreness or a low fever may appear afterwards — one sign the vaccine is working! — along with fussiness.

But how likely is it that your baby will have vaccine side effects, especially serious ones? Here's what you need to know about the nature and likelihood of your baby having a reaction to a shot.

Are vaccine side effects possible?

Vaccines are safe, but like any medicine, they do sometimes come with the possibility of a few side effects. Most of the time, these reactions to vaccines are mild, localized — there's some soreness or redness where the shot is given — and brief, lasting no more than a day or two. A low-grade fever may also occur, but anything more serious is very rare.

Proving that a vaccine is related to a specific side effect isn't easy. Babies and toddlers tend to get sick frequently, so there's a chance your child could be coming down with something right around the time the vaccine is given. That's why some parents may believe that their baby's vaccine caused the illness, when in reality germs picked up at day care, the store or a playdate are to blame.

It's also important to understand that vaccines are given at a time in life when certain health conditions begin or become apparent. For example, the MMR vaccine is given at a year to 15 months, the same time that developmental delays are often first noticed in a child, causing some parents to have questions about the relationship between the vaccine and any delays.

Vaccination Myths

What should I do if my baby has vaccine side effects?

A cold pack applied after the shot can help reduce soreness or swelling, as can moving the arm or leg in which the shot was given. You can "bicycle" a baby's legs to help bring relief, or encourage a toddler to do a few rounds of head-and-shoulders-knees-and-toes.

A non-aspirin pain reliever such as acetaminophen (age 2 months and older) or ibuprofen (6 months and older) can help with residual achiness and low-grade fever, so ask the doctor about that before you leave the office.

Is there any mercury in vaccines?

Very small amounts of thimerosal, an organic compound mercury-based preservative, used to be found in some vaccines starting in the 1930s to prevent bacterial and fungal contamination. Thimerosal is also currently found in some commonly used medicines and products such as contact lens solutions and throat and nasal sprays, although many manufacturers have reformulaed their products without thimerosal.

Even though several studies have shown that the very low levels of thimerosal once used in some vaccines didn't cause harm and the preservative was quickly expelled from an infant's body, experts decided that limiting children's exposure to any form or amount of mercury was a smart idea.

Accordingly, all routinely recommended vaccines made for infants in the U.S. have been thimerosal-free since 2001, except for the flu shot (and even then, only in multi-dose vials), which contains extremely small amounts that are expelled rapidly by the body.

What's more, many recommended childhood vaccines (MMR, polio, varicella and pneumococcal conjugate) never contained thimerosal at all.

Most importantly, numerous large-scale studies have not supported the theory that any ingredients in vaccines can cause developmental delays or disorders.

Can vaccines cause autism?

No, vaccines do not cause autism. Numerous studies involving children have found no link between vaccines and developmental disorders, including autism. A report from the Institute of Medicine based on comprehensive data and evidence gathered over a number of years also found no link between thimerosal or the MMR vaccine and developmental issues.

In fact, the long-disputed 1998 study that suggested a possible link between the MMR vaccine and autism was retracted in 2010 and has been rejected by all major health organizations, including the American Academy of Pediatrics (AAP), the Centers for Disease Control and Prevention (CDC), the National Institutes of Health (NIH) and the World Health Organization (WHO).

Since then, no other study has found a link between vaccines and autism or other developmental disorders, or between vaccines and cognitive and behavioral problems.

When to call the doctor about vaccine side effects

Though severe vaccine side effects are rare, you should call the pediatrician right away if your baby has any of the following reactions or symptoms within two days of getting a shot:

• High fever (100.4 degrees Fahrenheit or higher for babies under 3 months old and 101.5 or higher for babies 3 months and older)
• Crying for more than three hours at a time (not to be confused with colic) or a high-pitched cry that lasts more than one hour
• Seizures or convulsions — which may be related to a very high fever
• Severe, persistent seizures or major alterations in consciousness
• Listlessness, unresponsiveness or excessive sleepiness
• An allergic reaction such as swelling of mouth, face or throat; breathing difficulties; or a rash — which requires immediate medical attention and a call to 911
• Behavior changes — anything that's not normal for your child warrants a call to the pediatrician just to be on the safe side

Though these side effects could be unrelated to the vaccine and could be signs of an unrelated illness, call your doctor immediately if your child has any of these more serious symptoms (or others not on the list).

If your doctor believes your little one is having a reaction to a vaccine, he or she (or you) should report it to the Vaccine Adverse Event Reporting System (VAERS) at 1-(800) 822-7967 or vaers.hhs.gov. Experts analyze VAERS data and look for patterns that could indicate any possible safety issues with vaccines.