An observational study was conducted among 827 older adults aged ≥75 years in Hong Kong. Sera were collected immediately before and one month after receipt of the 2015/16 quadrivalent inactivated influenza vaccine. We measured antibody titers with the hemagglutination inhibition assay, and compared mean fold rise in titers from pre- to post-vaccination, and the proportion with post-vaccination titers ≥40 and ≥160.
Immune responses to influenza vaccination can be weaker in older adults than in other age groups. We hypothesized that antibody responses would be particularly weak among repeat vaccinees when the current and prior season vaccine components are the same.
Overall, we found that reduced antibody responses in repeat vaccinees were particularly reduced among older adults who had received vaccination against the same strains in preceding years.
Participants who reported receipt of vaccination during either of the previous two years had lower mean fold rise against all strains compared to those who were not. Mean fold rises for A(H3N2) and B/Yamagata were particularly weak following repeated vaccination with the same vaccine strain, but we did not generally find significant differences in the proportion of participants with post-vaccination titers ≥40 and ≥160.