Mucosal immunity as a vaccine-induced correlate of protection against influenza.

Licensure of influenza vaccines relies on serum hemagglutination inhibition (HAI) titers, a correlate of protection (CoP) that was developed more than 50 years ago and which is only poorly predictive of protection. This is especially true of immunity induced by intranasal live attenuated influenza vaccines (LAIVs). Unlike intramuscular inactivated influenza vaccines (IIVs), LAIV and natural infection selectively stimulate mucosal immunity. Developments in mucosal immunology now enable measurement of diverse aspects of mucosal immunity, including the frequencies and functions of nasal antibodies, T cells, and B cells. This review assesses the potential of new sampling and assay approaches that may overcome inconsistent findings from previous methodologies. Standardization of the collection and assessment of mucosal samples is essential in developing new CoPs for vaccine development and licensure of novel vaccines that induce nasal protection and are more effective in prevention of viral transmission.