Febrile Seizures After Childhood Vaccines Are Rare, Study Finds

A seizure caused by a fever in a young child can be terrifying, and some parents worry that the occasional fever that can follow a vaccine may cause one. But febrile seizures after vaccines are rare, a study finds, affecting 3 children out of 10,000. And children almost always recover completely.

The study, published Monday in Pediatrics, found that only a few vaccines or vaccine combinations increase the risk of febrile seizures. The pneumococcal vaccine given alone increases the risk, and so does the flu vaccine if given at the same time as either the pneumococcal vaccine or DTaP (diphtheria, tetanus, pertussis) vaccine. The pneumococcal vaccine, or PCV, protects against pneumococcal disease, which can range from an ear infection to pneumonia or meningitis, and kills 1 in 15 children under 5 who develop it.

"The basic message is, while there is an increase compared to giving them on separate days, the absolute amount of that risk is so, so very small that for the average person, it won't really affect their chances of having a febrile seizure," lead study author Jonathan Duffy tells Shots.

Febrile seizures, he adds, are considered isolated, benign events with no long-term consequences in the vast majority of children. They occur in approximately 2 to 5 percent of children, caused infections or other conditions that can cause a fever, according to Mark Sawyer, a professor of clinical pediatrics at University of California San Diego School of Medicine and Rady Children's Hospital in San Diego.

"For anybody who's seen a febrile seizure, it is a scary experience," Sawyer says. "People are not responding, sometimes they are not breathing very effectively, and there's uncontrolled twitching of arms and legs, so people assume there must be something major going on in the brain, but fortunately, that's not the case."

Febrile seizures occur most typically in children between 1 and 2 years old — the same ages when children receive a number of vaccines. Sometimes other seizures may follow if the child has a disorder such as epilepsy, but no evidence has suggested that a febrile seizure can cause seizure disorders.

"If you're really concerned about febrile seizures, the smartest strategy is to protect your child against the infection with the vaccine," Mark Schleiss, director of pediatric infectious diseases at the University of Minnesota, told Shots. About 20 percent of children hospitalized for influenza, for example, experience febrile seizures, he says. "I find this paper reassuring about vaccine safety," he adds.

Duffy, a medical officer in the Centers for Disease Control and Prevention Immunization Safety Office, says they conducted the study to follow up on reports five years ago that found an increased risk of febrile seizures when the flu and pneumococcal vaccines were given the same day.

This study used the Vaccine Safety Datalink, a massive database that tracks millions of children receiving vaccines in nine health systems across the U.S. The very large population size — approximately 9.8 million for this study — ensures the highest quality data possible in assessing vaccine safety. It's the largest vaccine safety surveillance program of its kind.

The researchers identified all children between 6 and 23 months old who had a febrile seizure within 20 days of at least one vaccine between 2006 and 2011. They compared the number of seizures occurring the day of or after a vaccine with those occurring 14 to 20 days after the vaccine in the same children. The first 24 hours is the time a febrile seizure caused by inactivated vaccines is biologically possible.

The researchers skipped days 2 to 14 because febrile seizures from live vaccines, such as the measles-mumps-rubella and chickenpox vaccines, are biologically likely then. The risk of a febrile seizure from these vaccines is already known to be 1 in 3,000 doses.

Among 333 febrile seizures that occurred following nearly 2 million vaccinations, half the children had had a previous febrile seizure. Those not caused by a vaccine resulted from respiratory tract infections, urinary tract infections, ear infections and viral infections.

A slightly increased risk of 3 febrile seizures per 10,000 children existed when receiving the PCV vaccine by itself, the inactivated trivalent influenza vaccine with either the PCV or DTaP vaccine, or all three vaccines together. A commentary accompanying the study, co-authored by Sawyer, calculated that this risk translates to approximately 1 child with a vaccine-caused febrile seizure every 5 to 10 years in the practice of an average pediatrician, seeing 1,000 children under 5 each year.

"While we've done all this work to quantify this risk, what we've found is that even at the highest risk we've seen [it] is still very small in absolute terms," Duffy says. "The pneumococcal vaccine prevents life-threatening conditions such as blood infections and meningitis, so the benefits of getting that vaccine are so much greater than the very small risk of getting a febrile seizure, which does not have the potential to cause long-term damage such as those infections can."

Though parents of children with a history of febrile seizures might want to discuss separating these vaccines with their child's doctor, that's not the wisest plan overall, Sawyer told Shots.

"What we know from many years of experience is that every time you skip a vaccine, a certain percentage of those kids will never get that vaccine or the doctor doesn't realize they didn't get that vaccine," Sawyer says. "Parents are going to naturally assume 'That's not me, of course I'll bring my child back,' but not everybody does, and then we have susceptible children in our community." Separating these vaccines also leaves children susceptible to those illnesses for longer, he added.

"It's kind of like not wearing a seat belt for two weeks," Sawyer says. "If you end up in a car accident in those two weeks, you're going to wish you had been wearing your seat belt."