The vaccine was administered after being diluted by the Pfizer COVID-19 vaccine diluent, 2.2 mL of 0.9% sodium chloride (normal saline, preservative free) of the diluent used for Pfizer COVID-19 vaccine into the bottle of magenta border Moderna; vaccine was administered, 0.2 mL dose and administered to 3 year 2 month child; This spontaneous case was reported by a nurse and describes the occurrence of PRODUCT PREPARATION ERROR (The vaccine was administered after being diluted by the Pfizer COVID-19 vaccine diluent, 2.2 mL of 0.9% sodium chloride (normal saline, preservative free) of the diluent used for Pfizer COVID-19 vaccine into the bottle of magenta border Moderna) and UNDERDOSE (vaccine was administered, 0.2 mL dose and administered to 3 year 2 month child) in a 3-year-old male patient who received mRNA-1273 (Moderna COVID-19 Vaccine) (batch no. AS1416B) for COVID-19 prophylaxis.     Patient did not had any allergies related to food, medications and other products. Patient had tested positive for Covid-19. Patient did not had any acute illness at the time of vaccination and up to one month before. Patient did not had any chronic or long standing health conditions.     On 08-Jul-2022 at 8:00 AM, the patient received first dose of mRNA-1273 (Moderna COVID-19 Vaccine) (Intramuscular) .2 milliliter. On 08-Jul-2022 at 8:00 AM, the patient experienced UNDERDOSE (vaccine was administered, 0.2 mL dose and administered to 3 year 2 month child). On an unknown date, the patient experienced PRODUCT PREPARATION ERROR (The vaccine was administered after being diluted by the Pfizer COVID-19 vaccine diluent, 2.2 mL of 0.9% sodium chloride (normal saline, preservative free) of the diluent used for Pfizer COVID-19 vaccine into the bottle of magenta border Moderna). At the time of the report, PRODUCT PREPARATION ERROR (The vaccine was administered after being diluted by the Pfizer COVID-19 vaccine diluent, 2.2 mL of 0.9% sodium chloride (normal saline, preservative free) of the diluent used for Pfizer COVID-19 vaccine into the bottle of magenta border Moderna) and UNDERDOSE (vaccine was administered, 0.2 mL dose and administered to 3 year 2 month child) outcome was unknown.        For mRNA-1273 (Moderna COVID-19 Vaccine) (Intramuscular), the reporter considered PRODUCT PREPARATION ERROR (The vaccine was administered after being diluted by the Pfizer COVID-19 vaccine diluent, 2.2 mL of 0.9% sodium chloride (normal saline, preservative free) of the diluent used for Pfizer COVID-19 vaccine into the bottle of magenta border Moderna) and UNDERDOSE (vaccine was administered, 0.2 mL dose and administered to 3 year 2 month child) to be not related.    No concomitant medications were reported. Patient age was reported as 3 years 2months. It was reported that the vaccine was administered to 2 children under the age of 4 years, The patients did not experience any side effects at that moment. The event did not caused patient to seek medical care. Patient had not experienced any similar event in the past. No treatment medications were reported.   Most recent FOLLOW-UP information incorporated above includes: On 08-Jul-2022: The recent live follow-up received on 10-JUL-2022 contains fax number and narrative were updated. On 11-Jul-2022: Follow up received contains non significant information. On 18-Jul-2022: Follow up contains significant information: Additional reporter added. Patient demographics added. Suspect product details updated. Events details updated. I-narrative supplement updated.