Previous changes to the COVID-19 vaccine schedule have been enabled through immediate modification orders, or IMOs, under the Epidemic Preparedness Act. Earlier this year, as the highly transmissible Omicron variant reached our shores, the Government used an IMO to make provision for third or "booster" doses of the Pfizer COVID-19 vaccine to be administered at a reduced dose interval of three months instead of the approved six months; this was done in order to maximise protection at that time.