However, HAI titer appears to be less applicable as a correlate of protection in high-risk populations (e.g., older adults) for developing severe influenza, and alternative markers of humoral (neutralizing antibodies, influenza virus-specific B cell ELISPOT response) and/or cellular immunity (granzyme B levels) have been suggested to more efficiently measure vaccine response and/or complement HAI response to better predict vaccine efficacy in this age group [18,19,42,44,45]. This evidence concerns the gene GZMB and influenza.