May 19, 2014
By Will Boggs MD
NEW YORK - Delaying early childhood vaccination, especially measles-containing vaccines, past 15 months of age is associated with an increased risk of postvaccination seizure, according to findings from the Vaccine Safety Datalink.
"The best way to protect children from serious diseases such as measles (which can kill) and to minimize side effects from vaccination is to vaccinate on time," Dr. Simon J. Hambidge from Kaiser Permanente Colorado and University of Colorado in Denver told Reuters Health by email.
At least one earlier study has shown that measles-mumps-rubella (MMR) and measles-mumps-rubella-varicella (MMRV) vaccines are associated with a higher risk of seizures if received after 15 months of age.
Dr. Hambidge and colleagues used data from the Vaccine Safety Datalink to examine risk for seizures after vaccination in young children in the first two years of life.
About half of the more than 5,600 children included in the study were vaccinated on time in the first two years of life for all vaccines, the researchers report in Pediatrics, online May 19.
For children who received their first infant vaccines at the recommended age of 38 to 92 days, there was no association of vaccination with seizures, but delaying immunizations beyond that point was generally associated with a nonsignificant elevation of the seizure risk.
For the MMR vaccine specifically, administration according to recommendations at 12 to 15 months of age was associated with a 2.65-fold increased risk of seizures in the seven to 10 days after vaccination. When administration was delayed past 15 months, this risk increased to 6.53-fold.
Results were similar for the MMRV vaccine: on-time administration was associated with a 4.95-fold increased risk of seizures in the subsequent seven to 10 days, compared with a 9.8-fold increased risk when vaccine was administered between 16 and 18 months of age.
The risk of febrile seizures is just over 1 per 100,000 person-days at age seven months and reaches a maximum of almost 5 per 100,000 person-days at age 17 months before declining to 1 per 100,000 person-days by age 45 months, the researchers note.
"In the second year of life, MMR vaccine causes approximately 1 case of simple febrile seizure for every 4,000 doses (this number is based on prior work, as our study did not quantify the risk per dose)," Dr. Hambidge said. "These seizures, although scary for parents, are temporary and do not result in any long-term health problems such as epilepsy (this has also been shown in previous work). The risk of catching a serious disease (measles) is far greater than the rare risk of seizures after MMR vaccine."
Dr. Dario Pruna, who heads the epilepsy unit at University Hospital in Cagliari, Italy, said the new work confirms earlier data but needs replication "in different populations with different vaccination schedules and different national health services."
He stressed in an email to Reuters Health that vaccines do not cause epilepsy, despite the increased risk of febrile seizures, and he pointed out that there is no "scientific rationale for delaying the vaccination schedules."
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