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.
The reactions, which occur a week to 10 days after vaccination, can be frightening, although they are typically non-fatal.
The New York Times reports:
"[Researchers] compared seizure and fever reactions among 83,107 1-year-olds who had combined MMR and chickenpox vaccinations with reactions of 376,354 toddlers who received separate vaccines."