Preexisting Immunity, Not Frailty Phenotype, Predicts Influenza Postvaccination Titers among Older Veterans.

Both preexisting immunity to influenza and age have been shown to be correlates of influenza vaccine responses. Frailty, an indicator of functional impairment in older adults, was also shown in one study to predict lower influenza vaccine responses among nonveterans. In the current study, we aimed to determine the associations between frailty, preexisting immunity, and immune responses to influenza vaccine among older veterans. We studied 117 subjects (age range, 62 to 95 years [median age, 81 years]), divided into three cohorts based on the Fried frailty test, i.e., nonfrail (NF) (n = 23 [median age, 68 years]), prefrail (n = 50 [median age, 80 years]), and frail (n = 44 [median age, 82 years]), during the 2010-2011 and 2011-2012 influenza seasons. Subjects received the seasonal trivalent inactivated influenza vaccine, and baseline and postvaccination samples were obtained. Anti-influenza humoral immunity, as measured by hemagglutination inhibition (HI) and microneutralization assays, was measured for influenza B, A(H1N1)pdm09, and A(H3N2) viruses. Postvaccination titers were not different between frail and NF subjects overall in this older subset of veterans. However, preexisting HI titers were strongly correlated with postvaccination titers among all functional status groups. When microneutralization titers were compared, the association between preexisting immunity and vaccine responses varied by frailty status, with the strongest correlation being observed for the NF group. In conclusion, preexisting immunity rather than frailty appeared to predict postvaccination titers in this older veteran cohort.