Client received the COVID19 Janssen vaccine (Lot#201A21A, EXP: 09/21/21) at 17:08.  The client was accompanied by a companion and after receiving the vaccine, the client and her friend went to the observation area to sit for 15 minutes.  At approximately 17:12, the client?s friend alerted Lead RN/EMT, something was wrong with the client.  Lead RN went to the client and observed her head tilted back with her eyes wide open in a blank stare, and her arms in the position of what appeared to be the clonic phase of a seizure, with no jerking movements.  Lead RN also observed the client?s chest moving up and down, to indicate she was breathing.  To determine if the client could respond, Lead RN asked the client if she was OK.  The client did not respond.  Lead RN pulled the client?s mask down and observed that she was foaming at the mouth.  Lead RN turned client?s head to the side and called for assistance from Co-Lead RN to get the client to the floor.  By the time the client reached the floor, Lead RN observed she had swallowed her saliva, her eyes were open and focused, and her arms were no longer in a flexed clonus position.  Rather, they were relaxed and normal.  Lead RN asked the client if she could tell her her name, to which the client responded, ?I can, but I?m scared so I don?t want to speak.?  Lead RN assured the client she was OK, and asked if she knew where she was.  The client stated she was at the vaccination site.  Lead RN also asked the client if she had a history of seizures and the client stated she had one in the past.  Then, she stated she has them when she gets stressed.  Lead RN observed that the client was diaphoretic and warm to the touch.  The client then stated she was thirsty, felt nauseous, and wanted to sit up.  Lead RN went to retrieve ice packs for the client while Co-Lead RN asked the client to grip both of his hands.  Co-Lead RN found that the client was able squeeze his hands and she exhibited equal strength in each limb.  At 17:14, the client sat up and vomited one time on the floor.  After doing so, the client requested water.  Lead RN provided the client with an ice pack for the back of her neck, which she was able to hold, placed a chux pad over the vomit, and provided the client with water, which the client immediately began to drink.  At 17:17, the client stood up and sat in the anti-gravity chair and an RN began taking her vitals (BP: 135/80, HR: 94, SPO2: 98%).  Co-Lead RN asked client questions and determined she was alert and oriented to person, place, time, and situation.  At that time, the client also stated she ?felt fine, like normal.?  Co-Lead RN asked the client if she had a primary care physician, to which she stated yes, and he recommended she follow up with him/her.  The client also inquired about booster shots for COVID19, and Co-Lead RN provided her with information.  Throughout the entire incident, the client?s friend was making belligerent remarks, stating ?something was put in the vaccine? and ?they are Regulatory Authority and now you can sue.?  The client had asked him to be quiet a couple of times.  While Co-Lead RN was answering the client?s questions on booster shots, the client?s friend voiced to the client that she did not need to provide any information and she did not have stay at the facility any longer.  After that, the client stood up and said she was fine and was leaving.  Co-Lead RN explained to the client she should consider staying longer for assessment and observation, but the client refused.  She stated she was fine, and at 17:22, she left against medical advice with her companion walking with a steady gait.