Participants will be adults aged 18 years or older who have been previously primed with two doses of either Pfizer-BioNTech (BNT162b2, or Comirnaty®) or AstraZeneca (ChAdOx1-S, or Vaxzevria®) and boosted at least 3 months earlier with Pfizer BioNTech vaccine (30µg). There will be no upper age limit. Participants will be recruited from the Murdoch Children's Research Institute, the Royal Children's Hospital (RCH), the Peter Doherty Research Institute, and, if necessary, the greater Melbourne area. Ideally, 100 participants will be recruited per group unless bivalent Omicron-specific vaccines are introduced before this target is reached for monovalent ancestral vaccines. There will be 800 participants in total. Procedures will be implemented to ensure participants of all ages (aged 18 and above) are included and that there is an even age distribution in each group (<50 and ≥50 years).
This clinical trial will be a blinded, randomised study to determine the safety, reactogenicity, and immunogenicity of a second booster dose of SARS-CoV-2 vaccines in adults enrolled over two consecutive stages. Stage 1 will commence at the time of study approval and transition to stage 2 once bivalent vaccines are approved and available in Australia.