Vaccinating children against measles when they are aged between 12 and 15 months is associated with less fever and fewer seizures during the 7 to 10 days after vaccination than is vaccinating between 16 and 23 months of age, but experts emphasize that the overall risk is low either way.
A retrospective cohort study involving over 840,000 children from 8 Vaccine Safety Datalink sites found that children who receive their first dose of any measles-containing vaccine at 16 to 23 months of age are roughly twice as likely to experience a seizure as those who receive their first dose at 12 to 15 months of age, but the risk for febrile seizures after any measles-containing vaccine is low—less than 1 febrile seizure per 1,000 injections.
Similarly, the risk of postimmunization fever was significantly greater among the older children than among the younger children, but the attributable risk was not.
When looking at different types of vaccine, the researchers found that the measles, mumps, rubella, and varicella (MMRV) vaccine was associated with a 1.4-fold increased risk of fever and a 2-fold increased risk of seizures, compared with the measles, mumps, and rubella (MMR) vaccine administered with or without varicella vaccine in both the younger and older age groups. In the 7 to 10 days after vaccination, this would translate into about 1 additional febrile seizure among every 2,000 children vaccinated with the MMRV vaccine, compared with children vaccinated with MMR and varicella separately at the same visit.
The Centers for Disease Control and Prevention recommends the first dose of MMR vaccine at 12 to 15 months of age.