The immunogenicity, reactogenicity, and safety of a bivalent mRNA or protein COVID-19 vaccine given as a fourth dose.

Objectives: We conducted a randomised controlled trial (RCT) to compare immunogenicity, reactogenicity and safety one month after a fourth COVID-19 mRNA or protein vaccine dose.

Methods: This RCT recruited healthy adults in Melbourne, Australia, who had previously received three COVID-19 vaccine doses at least six months prior and had no SARS-CoV-2 infection in the last three months. The participants were randomised (1:1) to receive the bivalent mRNA vaccine (mRNA-1273.