On 05/15/22 at approximately 11:09 AM while sitting in a chair in the observation area, 17 year-old female client fainted after receiving dose of Pfizer COVID-19 first booster. Vaccine was administered intramuscularly into client's left arm by nurse # 1 LOT #FK9895 at approximately 11:00 AM. At approximately 11:01 AM, when sitting in post-vaccination observation area next to her brother and father, Nurse # 2 noted client to be alert, awake, and oriented x 4 after asking client and her family, "do you feel okay here?" while offering client a bottle of water, which client accepted and verbalized that she "felt fine". At approximately 11:09 AM, client was observed by Nurse #3 and Nurse #2 to have fainted while sitting in chair, turned with her back not against the back of the chair. While fainting, client's upper torso became limp and her upper torso fell backward, hitting her head briefly on pavement. Nurse 2 and 3 quickly responded to client's side and both nurses supported client's upper body, while client's lower body was still seated in chair. Nurse 2 performed a sternal rub on client who opened eyes after approximately 2 seconds. At 11:10 AM, Nurse 2 and Nurse 3 lowered client to the ground in supine position, with a small pillow brought to support client's head. Visual assessment for apparent head injury/bleeding was performed by Nurse 3. Back of client's head appeared to be without bleeding and skin appeared dry, clean, and intact. At approximately 11:10 AM, Nurse #1 arrived to client's side to offer support and assist with scribing. At approximately 11:11 AM, Nurse 4 arrived to client's side to take vital signs. Vital signs at 11:11 AM were: HR 59, O2 98% Room air, RR 12, blood pressure was not taken at this time. Client was encouraged to remain lying down and layed on ground in supine position, with eyes open and was able to verbalize and answer questions from Nurse 3. Client reported that she did not eat anything for breakfast this morning. At 11:12 AM, Nurse 4 brought an icepack from first aid-kit to rest behind client's head. At 11:12 AM, Nurse 2 called 9-1-1 to request an ambulance to scene. At 11:24 AM, client remained AAO x 4, resting in supine position with Nurse 2 and Nurse 1 at client's side. At 11:24 AM, client's vital signs were taken by Nurse 4 as follows: HR 71, O2 Sat Room Air 96%, RR 12, BP 100/73. At 11:28 AM, AMR paramedics arrived and assumed care. AMR assessed client and took and reported client's blood glucose level as 105. Client's father refused for client be taken to hospital by AMR for further evaluation against medical advice. Client was assisted by AMR to a sitting position and then standing position at 11:32 AM. At 11:32 AM, AMR left the scene. At 11:33 AM, Nurse 3 explained signs and symptoms of confusion, nausea, concussion, or delayed or further head injury to client and client's father who both verbalized understanding. Nurse 2 recommended to client's father to follow up care with client's primary care physician the next day. Client's father verbalized understanding. At 11:35 AM, client was observed by Nurse 2 and Nurse 3 to walk away from vaccination site with steady gait, accompanied by client's father and client's brother. On 05/16/22, at 12:44 PM, Nurse 2 placed two courtesy follow up telephone calls to client's family at the phone number, listed on documentation system. There was no answer after both phone calls and Nurse 2 was unable to leave message, as automated message stated that "mailbox is full".