U.S. health officials are investigating an increase rise in reports of a type of seizure after vaccination with a Sanofi-Aventis SA flu vaccine, the Food and Drug Administration said on Thursday.

The febrile seizures, or seizures related to a fever, have been reported primarily in children younger than 2, the FDA said in a statement on its Web site.

Data are preliminary and the FDA and Centers for Disease Control and Prevention are investigating if they are associated with the vaccine, called Fluzone, or if other factors were involved, the FDA said.