At approximately 08:45hrs on 15 March 2021, vaccinators in Booth 1 reported to the on-site Pharmacy that they were on their second patient of the day but had no vaccine in the multi-dose vial. A rapid investigation was completed, and it was determined that the first patient who visited that booth today had received a dose from a vial that was potentially left-over from the clinic last night. An immediate conference with site leadership was conducted. The patient was identified via computer database by encounter time stamp and confirmed by the vaccinator team. Extensive research and consultation were conducted by Dr., including contacting Pfizer for consultation, and reviewing information from the Centers for Disease Control and Prevention (CDC) including Advisory Committee on Immunization Practices (ACIP) recommendations.  Per the Pfizer document entitled Storage and Handling of DILUTED VIALS Outside of Recommendations in the EUA Prescribing Information DURING and AFTER DILUTION?: Pfizer has conducted physical and chemical stability studies which have shown that the vaccine maintains all its measured quality attributes when diluted vaccine is stored in the vial at 2°C to 30°C for up to a cumulative period of 18 hours, in addition to the routine/labelled in-use period [of 6 hours]. It was determined that immediate administration of another dose was not required based on this information. In addition, clinical studies had not been performed that examined reissuing an initial vaccine after receiving one handled outside of the Emergency Use Authorization recommendations. The patient is scheduled to return in three weeks for their second dose. The patient was contacted and advised of the situation, and questions and concerns addressed. Immediate changes were implemented to avoid exhausted vials from being near the pharmacy.   Background:  Vaccinators in the clinic are from multiple agencies and are providing support to Community Vaccination Center Programs. Vaccine is reconstituted in the pharmacy. This is a controlled booth. Vaccinators enter the pharmacy booth at a defined entrance location where they receive a pre-filled plastic bin containing syringes, needles, vaccine record cards, and vaccine. The vaccine they receive is a multi-dose vial containing six doses, which are drawn-up individually in front of the patient in the vaccination booths due to community trust issues previously identified. When the supplies/vaccine is exhausted, the vaccinator returns to the pharmacy booth with their bin containing just the empty vial. This vial is stored in a plastic bag on the pharmacy table based off of inherited practices. The bin is replenished, and the vaccinator returns to their respective vaccination booth.   Incident:  Around 08:45hrs on 15 March 2021 vaccinators in Booth 1 reported to the joint on-site Pharmacy and indicated that upon draw-up for their second patient there was no vaccine in the multi-dose vial.  Investigation:  Dr. immediately convened a conference with the pharmacists present in the pharmacy booth. They included a Pharmacist, and Commanders.  It was determined that the vial likely originated from the bag of empty vials contained in a bag which had overflowed onto the pharmacy table. The vial in question likely contained an extra dose that was not administered last night due to appointment no-shows. This yielded the concern that the dose that was administered had become inactivated due to time after reconstitution and temperature. The vaccine was reconstituted at approximately 19:00hrs last evening (14 Mar 2021) and was no longer temperature controlled after that time.  Dr. contacted the manufacturer, Pfizer, who identified that their research indicates the vaccine to be chemically intact over 18 hours after reconstitution if stored in the vial between 2-30 degrees Celsius; this being the case for this vial. Dr. conferenced with the pharmacists, compared information with the CDC guidelines, and developed consensus that readministering the first dose was not required given that all quality attributes were likely maintained per the Pfizer stability studies. Dr. then notified the patient of the conclusion and provided instruction to return at their scheduled appointment date in three weeks for their second dose. Patient reported no symptoms at the time.     Immediate Actions Taken:  Department of Health utilized their Database to determine the patient who received the inactivated dose.   Dr. contacted the vaccine manufacturer Pfizer   Dr. then contacted the patient,   Controls were altered to remove the exhausted vials from the pharmacy table.   Vaccinators were instructed to verify their vials for appropriate volume/dosages.  Notifications made to site.  Patient advised to return to clinic at scheduled appointment time in three weeks to receive second dose.  Conclusion: The new safety controls in place in the pharmacy will prevent future similar occurrences. This information will be pushed to all clinic personnel in the form of a safety briefing to acknowledge the misstep and create awareness to decrease chances of future events. The patient was not harmed in this event and has received guidance from Dr. after review of manufacturer and CDC information.