Response rate of participants producing cytokines after recombinant severe acute respiratory syndrome coronavirus 2 spike glycoprotein (rS) stimulation, assessed by enzyme-linked immunosorbent spot assay (ELISpot). The response rate, defined as the proportion of participants in each treatment group at that time point with a count that exceeds the 95th percentile of pooled day 0 counts, was determined for each group, assessed by ELISpot. Response rate is plotted, shown as a percentage, for interferon gamma (IFN-γ; A), tumor necrosis factor alpha (TNF-α; B), IFN-γ and TNF-α (C), or interleukin 5 (IL-5; D). Group A: placebo (day 0/7, n = 31; day 28, n = 29). Group B: 2-dose 5 µg rS + 50 µg Matrix-M adjuvant (n = 32, n = 31). Group C: 1-dose 5 µg rS + 50 µg Matrix-M adjuvant (n = 28, n = 25). Group D: 2-dose 25 µg rS + 50 µg Matrix-M adjuvant (n = 33, n = 30). Group E: 1-dose 25 µg rS + 50 µg Matrix-M adjuvant (n = 31, n = 28).