Immunological evaluation of an mRNA vaccine booster in individuals fully immunized with an inactivated SARS‐CoV‐2 vaccine

According to clinical monitoring for 28 days after boosting, mild (grade 1) to moderate (grade 2) pain, redness, swelling and pruritus at the injection site were reported as the main local adverse events over 7 days. SEE PDF] Clinical trials of inactivated vaccines report antigen-specific humoural and cellular immune responses and protective efficacy against pandemics caused by the Wuhan strain.7,8 However, subsequent studies on the duration of immunity elicited by the vaccine against the Wuhan strain and other variants indicate dramatic declines in neutralizing and binding antibody titers, suggesting that a booster vaccination is needed to maintain neutralizing antibodies in this vaccinated population.9 In this study, a dose of 25 μg of mRNA vaccine as a clinical-stage product was used for booster immunization in individuals previously immunized with an inactivated vaccine, eliciting robust enhancement of antigen-specific antibody titers and cellular immune responses, which is similar to the data that heterologous boosting is associated with more robust immune responses than homologous boosting.10 Indeed, they were substantially greater than those achieved with booster immunization with the inactivated vaccine in our previous work and, importantly, also exhibited cross-reactivity with an antigen carrying the mutations of the delta and omicron variants, showing not only positive specific effective T cell responses against S antigens with various mutations but also high neutralizing antibody titers against the B.1.617.2 and B.1.1.529 and Wuhan strains. [...]all of these data support the conclusion that boosting previously inactivated vaccine-immunized individuals with an mRNA vaccine is a promising strategy for increasing population-level immunity against the rising pandemic caused by variants.