7 Ad26 vectors were produced expressing I). tPA.S Tissue plasminogen activator (tPA) leader sequence with full S; II). tPA.S.PP (2 proline substitutions); III). S. Wild type leader with full S; IV). S.dCT Wild type leader with deleted C-terminal of S; V). tPA.WT.S tandem tPA and full S; VI). S.dTM.PP Wild type leader with deletions in transmembrane and cytoplasmic regions; VII). S.PP Wild type leader with with 2 proline substitutions and mutation at cleavage site. An empty Ad26 vector was used as a negative control. Transduction was successful and the presences of all recombinant antigens were detected from cell lysates in Western blot. Quantification of neutralizing antibody (NAb) responses was performed using live virus neutralization assay and pseudovirus neutralization assay. Unlike other inactivated virus and nucleic acid vaccines, adenovirus vectors can induce long lasting Nab responses upon a single dose. However, it would be possible that an additional dose can enhance immunogenicity. Among the 7 S antigen vectors, the Ad26-S.PP was able to induce strong Nab responses and provide full protection against COVID-19 infection, but was weak to induce T-cell responses for adaptive immunity. Further studies regarding to the T-cell responses and half-lives of Nab were still yet to be determined.