From the Moderna Phase I results published recently [1], I'd point out that most vaccine candidates are also performing antibody challenges and T-cell counting, to better characterize the effective immune response generated [2], beyond antibody production.
[2] Via assessment methods in the above: "pseudotyped lentivirus reporter single-round-of-infection neutralization assay (PsVNA) and by live wild-type SARS-CoV-2 plaque-reduction neutralization testing (PRNT)" & "T-cell responses against the spike protein were assessed by an intracellular cytokine–staining assay, performed on specimens collected at days 1, 29, and 43"
[1] https://www.nejm.org/doi/full/10.1056/NEJMoa2022483
[2] Via assessment methods in the above: "pseudotyped lentivirus reporter single-round-of-infection neutralization assay (PsVNA) and by live wild-type SARS-CoV-2 plaque-reduction neutralization testing (PRNT)" & "T-cell responses against the spike protein were assessed by an intracellular cytokine–staining assay, performed on specimens collected at days 1, 29, and 43"