Administering the combination vaccine for measles, mumps, rubella (MMR) and chickenpox (varicella, or V) rather than giving the varicella shot separately on the same day may double the risk of febrile seizure in children.

In a study to be published in Pediatrics, researchers analyzed health records from children aged 12 to 23 months getting their first dose of measles-containing vaccine. A twofold increased risk of fever and febrile seizures seven to 10 days after vaccination was noted compared with same-day administration of separate shots for MMR and varicella. Vaccination with MMRV resulted in one additional febrile seizure for every 2,300 doses given instead of separate MMR+V. These brief, fever-related convulsions do not lead to epilepsy or seizure disorders.

Lead investigator Nicola Klein, MD, PhD, clarified that fewer than one febrile seizure occurs per 1,000 injections of measles-containing vaccine, but since the risk is higher for the combination MMRV vaccine, “Providers recommending MMRV should communicate to parents that it increases the risk of fever and febrile seizure over that already associated with measles-containing vaccines.”

The CDC recommends either MMRV or MMR+V for children aged 1 to 2 years; Dr. Klein advises MMR+V for families without a strong preference.