BackgroundThe ComFluCOV trial tested the safety and immunogenicity of COVID-19 and influenza vaccines co-administration. Binding and functional SARS-CoV2 anti-spike responses were measured using assays developed in response to the COVID-19 pandemic. The three assays used to measure the immunogenicity outcomes are reported here and their performance compared to inform future vaccine development.MethodsAdults aged over 18 were vaccinated with a COVID-19 and either an influenza vaccine or saline placebo. Serum sampled one month after vaccination was used to measure SARS-CoV2 anti-spike antibody concentrations using a commercial in-house enzyme-linked immunosorbent assay (ELISA), a commercial fast throughput electrochemiluminescence immunoassay (ECLIA) and a viral neutralisation assay (VNA). Geometric mean ratios were used to compare the response to COVID-19 with or without influenza vaccine with a threshold of 0.67 considered non-inferior. The relationship between the different assays was examined using Kendall rank correlations.ResultsThe geometric mean ratios exceeded 0.67 using all assays for all COVID-19 and influenza vaccine combinations tested. Moderate rank correlations were found between the three assays.ConclusionAll three assays confirmed that vaccine co-administration did not significantly impact on immunogenicity of any of the vaccines tested.Trial registrationISRCTN14391248, registered on 17/03/2021.