Two individuals were each given a full vial of the undiluted Pfizer-BioNTech vaccine at a clinic in Hougang on Sept 15.
Responding to queries, the Ministry of Health (MOH) said on Tuesday that one of them was hospitalised after experiencing a headache and an increased heart rate and has since been discharged.
The other patient did not report any adverse reaction.
The MOH said it was alerted to the incident on Sept 19 and that both individuals were given the vaccine at ProHealth Medical Group @Hougang.
Each full vial of the undiluted vaccine contains five doses’ worth.
Said the Health Ministry: “MOH takes a serious view of this incident and is carrying out a thorough investigation.
“The clinic and doctor who administered the vaccine have been suspended from the National Vaccination Programme until further notice.”
The Straits Times has asked MOH when the hospitalised patient was discharged, and what members of the public should do if they suspect they were given an undiluted dose of the vaccine.
There have been previous incidents of the wrong Covid-19 vaccine dosage being administered to patients.
A staff member at the Singapore National Eye Centre was given five doses of the Pfizer-BioNTech vaccine in one injection on Jan 14 last year.
The centre said later that the mistake arose from human error after a lapse in communication among members of the vaccination team.
It said the worker in charge of diluting the vaccine had been called away to attend to other matters before it was done.
A second staff member had then mistakenly thought the undiluted dose in the vial was ready to be administered.
The error was discovered within minutes of the vaccination.
In another incident, 117 patients and staff at Bukit Merah Polyclinic were given around one-tenth the recommended dose between Oct 20 and Oct 22 last year, requiring replacement doses.
In this case, the error was reportedly the result of a mistake in identifying the markings on new syringes.