Reactogenicity, Safety, and Serological and Cellular Immunogenicity of a Booster Dose of SARS-CoV-2 mRNA Prototype, Variant, and Bivalent Vaccines

Waning immunity after two SARS-CoV-2 mRNA vaccinations and the emergence of variants precipitated the need for booster doses. We evaluated safety and serological and cellular immunogenicity through 6 months after a third mRNA vaccination in adults who received the mRNA-1273 primary series in the Phase 1 trial approximately 9 to 10 months earlier. The booster vaccine formulations included 100 mcg of mRNA-1273, 50 mcg of mRNA-1273.351 that encodes Beta variant spike protein, and bivalent vaccine of 25 mcg each of mRNA-1273 and mRNA-1273.351. A third dose of mRNA vaccine appeared safe with acceptable reactogenicity. Vaccination induced rapid increases in binding and neutralizing antibody titers to D614G, Beta, Delta and Omicron variants that persisted through 6 months post-boost, particularly after administration of Beta-containing vaccines. Spike-specific CD4 + and CD8 + T cells increased to levels similar to those following the second dose. Boost vaccination induced broad and durable humoral and T cell responses. ClinicalTrials.gov numbers NCT04283461 (mRNA-1273 Phase 1) and NCT04785144 (mRNA-1273.351 Phase 1)