Patient immunized in a sitting position in arm and did well with vaccine, reported that vaccine did not hurt. Patient moved chairs at that time to allow for brother to take a seat in chair as well to start his vaccine,patient was in another chair in a room at that time. I was headed up to clean up vaccine and finish paperwork for child and patient slid down chair and fell forward and eyes up at that time, patient appeared to bump left side of head.   NP jumped to the floor at that time and held child to support neck and patient with gurgle sound in mouth and had decerbrate posturing and eyes rolled back with slight shake to arms in a rigid position. Clinical appearance with seizure vs vagovagal response. Patient with eyes rolling, rigidity and posturing and slight shake and oxygen level immediately obtained that time and dropped in the 70 range, episode lasted approximately 30 seconds. After episode developed facial pallor, severe fatigue and very tired. NP on floor with patient and NP student in room helping with obtaining vitals and assisting NP, Mom called EMS and older brother also in room.   Patient very tired and wanted to sleep. NP had patient flat on table at this point as awaking for EMS arrival, oxygen level back up in high 90s and breathing and speaking once again. Glucose obtained at this time and was 93 in clinic. (lot# MW3320S, exp 11/20/2020) Police arrived on site about 10 minutes prior to EMS. 2 groups then came via EMS. Neuro exam completed on patient and was able to follow EOMS and pupils equal and reactive to light after episode, speech is not slurred at this point but patient with sweating and facial pallor. Patient with no signs of angioedema, no hives, airway open as patient now speaking.   Mom does report patient is following with neurology at hospital for arachnoid cyst along with DVA in which he had MRI back in may, also follows with neurology as patient has had migraine history, mom reports starring spell and possible seizure at age 2 in which he started following at hospital.  EMS arrived obtained a brief history and then took the patient via ambulance, I updated EMS and I took older brother out of room and discussed baseball to avoid scaring him with the whole situation and mom stayed with patient. Patient taken from clinic at 3:50 PM, worker from EMS arrived back at 4:20 and reports father requesting type of vaccine given and wanted to ensure it did not have eggs. Immunization report provided to EMS worker and showed box and lot of vaccine and confirmed it did not have egg component. Dad did not come back to clinic and NP went into other room on B to provide EMS with the rest of information needed as I was actively seeing patients. Dad told older son so mom could be with patient in ambulance.   Mom signed paper for care everywhere to be pulled, documentation from MD monitoring headaches and also neurosurgery reviewed. Patient with increasing headaches recently from 2 a month to weekly frontal headaches. Patient taken to hospital in which doctors who already know his case are there, has been getting flu vaccines yearly but monitored in allergist office due to egg allergy. Mom reports patient started Xopenex this this week for asthma which was new for him.    Detailed medical history obtained after vaccine administered per mom during event and was not part of routine vaccine screening. Patient will be called to check in on patient and see of status after ER transport and is scheduled for a call back .  This may not have at all been related to vaccine or administration as safety precautions maintained and patient was sitting for administration and after, but will contact VAERS due to this being after vaccine.