More Shots on Goal: Heterologous Influenza Prime-Boost Vaccination Decreases the Likelihood of a Negligible Antibody Response to Current or Historical Variants

Seasonal influenza vaccination only elicits a strong and durable antibody response in a subset of recipients, and prior homologous booster trials have failed to improve on single-dose vaccination. Here, we conducted a phase 4 clinical trial to test if a heterologous prime-boost increases antibody magnitude or breadth as well as reduces the rate of non-responders (ClinicalTrials.gov ID# NCT06567860 ). Participants received a late-season 2023 inactivated influenza vaccine followed by the 2024 booster four months later containing an updated H3N2 strain alongside unchanged H1N1 and B/Victoria strains, allowing within-subject comparison of heterologous and homologous boosting. The booster vaccination halted or reversed antibody titer waning for all vaccine strains, most notably restoring seroprotective and seroconverting titers for B/Victoria at the cohort-level. Critically, the heterologous booster decreased the rate of non-responders (exhibiting HAI≤10 at all time points), with 0% H3N2 non-responders among subjects receiving the prime and boost compared to 29% H1N1 non-responders. A similar pattern held in a larger cohort administering consecutive fall vaccinations, where non-response rate was 20% of H3N2 compared to 37% for H1N1. Across all non-responders that had consistently low titers against the vaccine strains, up to half elicited an antibody response against historical variants, suggesting that immune imprinting partially underlies the non-responder phenotype. Despite differing vaccination timing, late-season priming and standard fall vaccination produced comparable antibody breadth 30 days post-vaccination when cohorts were matched on pre-vaccination titers. These findings suggest that heterologous vaccinations or other multi-strain exposures can reduce the fraction of the population that does not mount an antibody response during an influenza season.