The combination vaccine for measles, mumps, rubella and chickenpox is associated with double the risk of febrile seizures for 1- to 2-year-old children compared with same-day administration of the separate vaccine for measles, mumps and rubella and the varicella vaccine for chicken pox. Funded by the US Centers for Disease Control, the study analyzed 459,000 children 1 to 2 years old from numerous health systems across the US receiving their first dose of measles-containing vaccine.