 [ Symptoms ] can the flu booster cause permanent behavior change in babies 14 months old [ Disease ] no the flu vaccine will not cause permanent or any behavioral changes but the age is significant at age 14 15 months is really the beginning of a period often termed as the terrible twos it starts in the child is second year like now not the second birthday some of my most challenging examinations are in children of this age this is a period that can be hallmarked by interesting tantrums misbehavior stranger anxiety even with people they know and assertion of their independence a very challenging time so take a deep breath be patient but consistent with your discipline it will all be over in a year or so