Two people were reportedly given a full vial of the undiluted Pfizer-BioNTech Vocid-19 Vaccine at an Hougang clinic last Thursday.
Each vial of the vaccine when undiluted is equivalent to about 5 doses.
The Ministry of Health told The Straits Times on Tuesday that one of the patients was hospitalised following symptoms of headaches and heart rate increase, but has since been discharged from the hospital.
The other affected patient was reported to have no adverse reactions from the full vial dosage.
MOH said that they were alerted to the incident on 19 September and that both of the affected patients were administered the vaccine by ProHealth Medical Group at Hougang.
MOH added that they take a serious view of the incident and is carrying out a thorough investigation, and they added that the clinic and doctor who administered the vaccine have since been suspended from the National Vaccine Programme until further notice.