The immune system is complex and has multiple mechanisms to recognize and attack pathogens; besides antibodies I've seen mention of multiple types of T-cells an

The immune system is complex and has multiple mechanisms to recognize and attack pathogens; besides antibodies I've seen mention of multiple types of T-cells and also there's apparently a dimension of mucosal immunity vs. systemic immunity. 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.