Background: Immune responses to influenza vaccination tend to be lower among older, frequently vaccinated adults. Use of egg-free influenza vaccines is increasing, but limited data exist on factors associated with their immunogenicity in older adults. Methods: Community-dwelling older adults ≥56 years of age were enrolled in a prospective, observational study of immunogenicity of 2018–2019 influenza vaccine. Hemagglutination inhibition (HAI) antibody titers were measured pre-vaccination (Day 0) and four weeks after vaccination (Day 28) to calculate geometric mean titers, seropositivity (HAI titers ≥1:40), seroconversion (four-fold rise in HAI titer with post-vaccination titer ≥1:40) and geometric mean fold rise (GMFR). Linear regression models assessed the association of predictors of GMFR for each vaccine antigen. Results: Among 91 participants who received egg-free influenza vaccines, 84 (92.3%) received quadrivalent recombinant influenza vaccine (RIV4, Flublok, Sanofi Pasteur), and 7 (7.7%) received quadrivalent cell culture-based influenza vaccine (ccIIV4, Flucelvax, Seqirus). Pre-vaccination seropositivity was 52.8% for A(H1N1), 94.5% for A(H3N2), 61.5% for B/Colorado and 48.4% for B/Phuket. Seroconversion by antigen ranged from 16.5% for A(H1N1) and B/Colorado to 37.4% for A(H3N2); 40 participants failed to seroconvert to any antigen. Factors independently associated with higher GMFR in multivariable models included lower pre-vaccination HAI antibody titer for A(H1N1), B/Colorado and B/Phuket, and younger age for A(H1N1). Conclusion: Overall pre-vaccination seropositivity was high and just over half of the cohort seroconverted to ≥1 vaccine antigen. Antibody responses were highest among participants with lower pre-vaccination titers. Among older adults with high pre-existing antibody titers, approaches to improve immune responses are needed.
【저자키워드】 Influenza, Adults, Humoral response, Hemagglutination inhibition assay, Egg-free influenza vaccine,