Published: June 28, 2010

Toddlers who get a vaccine that combines the measles-mumps-rubella and chickenpox immunizations are at twice the usual risk for fevers that lead to convulsions, a new study reports.

The risk for a so-called febrile seizureafter any measles vaccination is less than 1 seizure per 1,000 vaccinations; but among children who received the combined vaccine, there is 1 additional seizure for every 2,300 vaccinated, said Dr. Nicola Klein, the study’s lead investigator and director of the Kaiser Permanente Vaccine Study Center.

The reactions, which occur a week to 10 days after vaccination, are not life-threatening and usually resolve on their own. The fever-related convulsions can be frightening, but they are brief and not linked to any long-term complications or seizure disorders.

To do the analysis, published this week in the journal Pediatrics, Kaiser Permanente researchers used the government’s Vaccine Safety Datalink, a safety surveillance system that compiles data on nine million members of eight managed-care organizations.

They compared seizure and fever reactions among 83,107 1-year-olds who had combined M.M.R. and chickenpox vaccinations with reactions of 376,354 toddlers who received separate vaccines.

“Unless parents have a strong preference for the combination vaccine, providers should use a separate vaccine,” Dr. Klein said.

A version of this article appeared in print on June 29, 2010, on page D6 of the New York edition.