Maybe a stupid question: Is this the standard measurement of effectiveness? I.e. you give the control group a dummy vaccine, the test group the real vaccine, and you just wait for them to be infected? Why not test for antibody development?
Antibodies are easy to test for (the chemistry required can be put into mass-produced home test kits), while it's more difficult to test for other responses (a recent long-term study of covid-19 T-cell response was with only ~100 subjects).
A hypothetical vaccine that produced a meh antibody response but mobilized the T-cell or mucosal systems could be effective at preventing the disease but not look like much on an antibody screen.