Combining the vaccine for MMR (Measles, Mumps, Rubella) with varicella (chicken pox) increases a child’s chances of having a fever-related seizure seven to 10 days after inoculation, according to a new report by the Kaiser Permanente Vaccine Study Center.
But researchers say the risk to toddlers is still pretty low.
“What’s important for parents to understand is that even though there’s a doubling of the risk for the combination vaccine, the overall risk of seizure to any one child with any measles-containing vaccine is still less than one in 1,000 doses,” Dr. Nicola Klein, lead investigator of the study, told HealthDay.
The Centers for Disease Control and Prevention funded the study, while Vaccine Safety Datalink, a CDC-sponsored vaccine-surveillance system, provided researchers with the health records of more than 459,00 children who received a dose of measles vaccine between 2000 and 2008.
According to the data, children between one and two who received the combination vaccine had one additional seizure for every 2,300 doses when compared to children who received separate shots, HealthDay reports.
Febrile seizures are brief convulsions caused by fever that occur in about 5 percent of children between six months and five years. They are frightening to parents, but don’t lead to epilepsy or seizure disorders, Klein said.
Last month, the CDC recommended that children with a personal or family history of seizures get separate vaccines.

