In keeping with ATAGI advice, people 5 years old and over with impaired immune function or who are taking medications that impair immunity should have a third dose of a Covid-19 vaccine to complete their primary course. People 16 years and over should also have a fourth dose and 5th dose to act as their boosters (see below).
An mRNA vaccine (Pfizer or Moderna) is preferred to Vaxzevria (AstraZeneca) for this 3rd dose. AstraZeneca can be used for the 3rd dose for individuals who have received AstraZeneca for their first 2 doses if there are no contraindications or precautions for use, or if a significant adverse reaction has occurred after a previous mRNA vaccine dose which contraindicates further doses of mRNA vaccine (e.g., anaphylaxis, myocarditis).
The recommended interval for the 3rd dose is 2 to 6 months after the 2nd dose of vaccine. A minimum interval of 4 weeks may be considered in exceptional circumstances (e.g., anticipated intensification of immunosuppression; outbreaks).