Fig. 1

Experimental design and assessment of serum antibody titers. A) Experimental design. Patients were randomized to metformin (final dose 1500 mg ER/day) or placebo and started with 1 tablet/day for week 1 (500 mg metformin ER/day or placebo), then 2 tablets a day for week 2 (1000 mg metformin ER or placebo), and finally 3 tablets a day for week 3 (1500 mg metformin ER or placebo) until the completion of the study. Participants were vaccinated with high-dose flu vaccine after ~ 10 weeks of treatment. Blood was drawn prior to treatment (week -10, Baseline), immediately prior to vaccination (week 0), and 1-, 5-, and 10-weeks post vaccination. Serum was analyzed via hemagglutinin inhibition assay for flu antibody titers. Geometric Mean Titers (GMT) were normalized to pre-vaccination (Pre-Vax) levels to calculate fold change for the B) A/Kansas/14/2017 (H3N2)-like virus, C) A/Brisbane/02/2018 (H1N1)pdm09-like virus and D) B/Colorado/06/2017-like virus (B/Victoria lineage) contained in the seasonal flu vaccine. The dashed line denotes a fold change of 1. Statistical significance was calculated by two-way repeated measures ANOVA with Šídák’s posthoc corrections and significance set at p < 0.05