摘要Background Optimising the immunogenicity of COVID-19 vaccines to improve their protection against disease is neces-sary.Fractional dosing by intradermal(ID)administration has been shown to be equally immunogenic as intramuscular(IM)administration for several vaccines,but the immunogenicity of ID inactivated whole severe acute respiratory syndrome coronavirus 2(SARS-CoV-2)at the full dose is unknown.This study(NCT04800133)investigated the superiority of antibody and T-cell responses of full-dose CoronaVac by ID over IM administration in adolescents.Methods Participants aged 11-17 years received two doses of IM or ID vaccine,followed by the 3rd dose 13-42 days later.Humoral and cellular immunogenicity outcomes were measured post-dose 2(IM-CC versus ID-CC)and post-dose 3(IM-CCC versus ID-CCC).Doses 2 and 3 were administered to 173 and 104 adolescents,respectively.Results Spike protein(S)immunoglobulin G(IgG),S-receptor-binding domain(RBD)IgG,S IgG Fcγ receptor Ⅲa(FcγRⅢa)-binding,SNM[sum of individual(S),nucleocapsid protein(N),and membrane protein(M)peptide pool]-specific interleukin-2(IL-2)+CD4+,SNM-specific IL-2+CD8+,S-specific IL-2+CD8+,N-specific IL-2+CD4+,N-specific IL-2+CD8+and M-specific IL-2+CD4+responses fulfilled the superior and non-inferior criteria for ID-CC compared to IM-CC,whereas IgG avidity was inferior.For ID-CCC,S-RBD IgG,surrogate virus neutralisation test,90％plaque reduction neutralisation titre(PRNT90),PRNT50,S IgG avidity,S IgG FcγRⅢa-binding,M-specific IL-2+CD4+,interferon-γ+CD8+and IL-2+CD8+responses were superior and non-inferior to IM-CCC.The estimated vaccine efficacies were 49％,52％,66％and 79％for IM-CC,ID-CC,IM-CCC and ID-CCC,respectively.The ID groups reported more local,mild adverse reactions.Conclusion This is the first study to demonstrate superior antibody and M-specific T-cell responses by ID inactivated SARS-CoV-2 vaccination and serves as the basis for future research to improve the immunogenicity of inactivated vaccines.