Immunogenicity and Reactogenicity Following a Booster Dose of COVID-19 mRNA Vaccine (Pfizer-BioNtech) and Two Adjuvanted Sub-unit Vaccines (SP/GSK) Administered in Adults Who Received 2 Doses of Pfizer-BioNTech mRNA Vaccine as a Primary Vaccination

The vaccination campaign in France began in early 2021 and was declared mandatory for all French people in July 2021. The efficacy of COVID-19 vaccines has since been widely demonstrated, as well as their safety, and now 60% of the adult population in France has received a first dose, most of them with Pfizer-BioNTech's mRNA vaccine. However, despite the increasing coverage, new data highlight the need for a booster dose for the most vulnerable people, including patients with immune deficiency. This makes it likely that a booster dose will also be needed in the general population, especially among healthcare workers, due to the active circulation of new variants since the beginning of summer 2021 and evidence of reduced protection against them. On the other hand, in addition to evaluating the potential benefit of a booster vaccination, it appears interesting to also evaluate a heterologous vaccination regimen, i.e. a booster with a different vaccine than the one used for the primary vaccination. Some studies have already evaluated a two-dose heterologous regimen and the results have shown stronger protection against SARS-CoV-2. In addition, this alternative could provide a real benefit in terms of accessibility, cost, and acceptability. The vaccine developed by Sanofi-Pasteur is based on a traditional recombinant protein approach using GSK's AS03 adjuvant. Two formulations of this vaccine are currently under development, the first targeting the S protein of the D614 strain (Wuhan strain), the second targeting the B.1.351 variant. Their value as a booster needs to be evaluated. The objective of this trial is therefore to evaluate the immunological response and safety induced by a homologous vaccine booster (Pfizer-BioNTech vaccine booster) and a heterologous vaccine booster (one of the two experimental Sanofi/GSK vaccines booster), on the D614 (Wuhan) strain and on the SARS-CoV-2 variants.