Principle 7: T-cell responses may be correlates of protection
The numerous functional classes of T-cells are suspected to be correlates of protection despite limited knowledge about them.
CD8+ T-cell cytotoxicity is one of the most important protective functions. In addition, CD4+ T cells influence protection directly or indirectly through cytokine secretion.
Principle 8: More than one factor may protect as co-correlates of protection
Multiple factors may function as co-correlates of protection.
Example: The live, attenuated intranasal influenza vaccine generates both nasal IgA mucosal antibody and serum antibody. Vaccinated children who were challenged with another dose of the intranasal vaccine demonstrated three different responses:
Virus shedding happened at a high rate in vaccinees lacking both responses, at an intermediate rate if only one response was present, and at a low rate in vaccinees having both responses.
Therefore, the two responses were synergistic or at least additive.