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.