Background
Influenza remains a significant threat to human and animal health. Assessing serological protection against influenza has relied upon haemagglutinin inhibition (HAI) assays, which are used to gauge existing immune landscapes, seasonal vaccine decisions and in systems vaccinology studies. HAI assays were first described in the 1940s. Here, we adapt our high-throughput live virus microneutralisation (LV-N) assay for SARS-CoV-2, benchmark against HAI assays, and report serological vaccine responsiveness in a cohort of older (>?65?yo) community dwelling adults.

Methods
Influenza-specific antibody responses were assessed in 73 individuals, before and after receipt of the adjuvanted 2021–22 Northern Hemisphere quadrivalent vaccine. We performed both HAI and LV-N assays against all four viruses represented in the vaccine [A/Cambodia/e0826360/2020 (H3N2), IVR-215 (A/Victoria/2570/2019-like) (H1N1)pdm09, B/Phuket/3073/2013 (B/Yamagata lineage), B/Washington/02/2019 (B/Victoria lineage)], using sera drawn before vaccination [range: d-82 to d-5], and days 7 [d6–10] and 181 [d156–200] after vaccination. We compared serological responses within each assay and between assays.

Results
Both the traditional HAI assay and our high-throughput live virus microneutralisation identified vaccine-induced boosts in antibody titres. We found population-level concordance between the two assays (Spearman´s correlation coefficient range 0.49–0.88; all p?≤?1.4?×?10?5). The improved granularity of microneutralisation was better able to estimate fold changes of responses and quantify the inhibitory effect of pre-existing antibody.

Conclusions
Our high-throughput method offers an alternative approach to assess influenza-specific serological responses with improved resolution, with the potential to improve the annual assessment of existing antibody landscapes, to improve new vaccine strain evaluation, and to offer a step-change in systems vaccinology, and a facet of laboratory-based pandemic preparedness.