Booster; mistakenly used sterile water for injection instead of sterile sodium chloride as the diluent for Comirnaty or the Pfizer-BioNTech COVID-19 vaccine; mistakenly used sterile water for injection instead of sterile sodium chloride as the diluent for Comirnaty or the Pfizer-BioNTech COVID-19 vaccine; Headache; brain fog; Mild soreness at the injection site; This is a spontaneous report received from a contactable reporter(s) (Other HCP) from medical information team.  A 43 year-old female patient received bnt162b2 (BNT162B2), administered in arm left, administration date 03Jan2022 (Lot number: FE3594, Expiration Date: 03Jan2022) at the age of 43 years as dose 3 (booster) 0.3ml, single for covid-19 immunisation. Relevant medical history included: "Type 1 diabetes mellitus" (ongoing). There were no concomitant medications. Vaccination history included: Bnt162b2 (First dose , Lot: EL0140, Expiration: Unknown, Expiry Date of Covid-19 Vaccine: 03Jan2022), administration date: 21Dec2020, when the patient was 42 years old, for COVID-19 Immunization; Bnt162b2 (Second dose: 18Jan2021, Lot: Unknown, Expiration: Unknown, Expiry Date of Covid-19 Vaccine: 03Jan2022), administration date: 18Jan2021, when the patient was 42 years old, for COVID-19 immunization. The following information was reported: IMMUNISATION (non-serious) with onset 03Jan2022, outcome "unknown", described as "Booster"; PRODUCT PREPARATION ERROR (non-serious), POOR QUALITY PRODUCT ADMINISTERED (non-serious) all with onset 03Jan2022, outcome "unknown" and all described as "mistakenly used sterile water for injection instead of sterile sodium chloride as the diluent for Comirnaty or the Pfizer-BioNTech COVID-19 vaccine"; HEADACHE (non-serious) with onset 03Jan2022, outcome "unknown", described as "Headache"; THINKING ABNORMAL (non-serious) with onset 03Jan2022, outcome "unknown", described as "brain fog"; VACCINATION SITE PAIN (non-serious) with onset 03Jan2022, outcome "unknown", described as "Mild soreness at the injection site". Additional information: It was reported that the Nurse Practitioner calling on behalf of her Pharmacist that mistakenly used sterile water for injection instead of sterile sodium chloride as the diluent for Comirnaty or the Pfizer-BioNTech COVID-19 vaccine. 2 doses of the vaccine were administered as the Booster dose for a 43 and 37-year old female patient. Can I use the vaccine? If already used to vaccinate individuals, does this count as a valid dose? Pfizer has not tested the effects of using sterile water for injection as the diluent for Comirnaty or any of the Pfizer-BioNTech COVID-19 Vaccine formulations that require dilution (ie, purple or orange cap). The diluted vaccine using sterile water for injection as the diluent should be appropriately discarded and not used for vaccination. If the vaccine that was diluted with sterile water for injection was inadvertently administered to individuals, a definitive statement on the quality and usability of the vaccine diluted in water cannot be made. The vaccine has not been tested with sterile water for injection as the diluent. Pfizer is unable to definitively state if the vaccine dose is valid or invalid. However, the CDC states the following: & quot; Contact the manufacturer for information on the stability of the vaccine. If the manufacturer does not have information to support the stability of the vaccine, repeat the dose immediately (no minimum interval). & quot; Source: ACIP  Follow-up attempts are completed. No further information is expected.; Sender's Comments: Linked Report(s) : US-PFIZER INC-202200011184 Same reporter/ drug/ event for different patients.