Influence of pre-existing hemagglutination inhibition titers against historical influenza strains on antibody response to inactivated trivalent influenza vaccine in adults 50-80 years of age

Background: Concerns about influenza vaccine effectiveness in older adults and the role of influenza strains encountered earlier in life led to this study. Methods: Antibody responses against antigens in the 2011-2012 influenza vaccine at 21 days post vaccination were analyzed in 264 individuals aged 50-80 years. At Days 0 and 21, sera were tested for hemagglutination-inhibition titers against these vaccine strains and at Day 0 against a panel of 15 historical seasonal strains. Results: The proportions of participants with seroprotective titers ≥1:40 to the vaccine strains at Days 0 and 21, respectively, were 37% and 66% for A(H1N1) and 28% and 63% for A(H3N2). An increasing number of responses ≥1:40 against historical strains was associated with seroprotective responses after vaccination among participants with a titer <1:40 at Day 0 for A(H1N1) and A(H3N2) vaccine strains (P < 0.01). In multivariable regression analyses among those with Day 0 titer <1:40, after controlling for age, sex, race, site and diabetes, Day 21 titers ≥ 1:40 for the vaccine A strains were significantly more likely as the number of seroprotective responses against historical strains increased (A(H1N1) odds ratio [OR] = 1.41, 95% confidence interval [CI] = 1.09-1.82 and A(H3N2) OR = 1.32, 95% CI = 1.07-1.62). The likelihood of seroconversion was significantly higher with an increasing number of responses to historical strains for A(H3N2) only (OR = 1.24, 95% CI = 1.01-1.52). Seroconversion was significantly less likely as Day 0 vaccine strain titers increased. Conclusions: Seroprotective titers after influenza vaccination increased as the number of responses to historical strains increased.