The trial was initiated on May 26, 2020; 134 participants underwent randomization between May 27 and June 6, 2020, including 3 participants who were to serve as backups for sentinel dosing and who immediately withdrew from the trial without being vaccinated (Fig. S1). Of the 131 participants who received injections, 23 received placebo (group A), 25 received 25-μg doses of rSARS-CoV-2 (group B), 29 received 5-μg doses of rSARS-CoV-2 plus Matrix-M1, including three sentinels (group C), 28 received 25-μg doses of rSARS-CoV-2 plus Matrix-M1, including three sentinels (group D), and 26 received a single 25-μg dose of rSARS-CoV-2 plus Matrix-M1 followed by a single dose of placebo (group E). All 131 participants received their first vaccination on day 0, and all but 3 received their second vaccination at least 21 days later; exceptions include 2 in the placebo group (group A) who withdrew consent (unrelated to any adverse event) and 1 in the 25-μg rSARS-CoV-2 + Matrix-M1 group (group D) who had an unsolicited adverse event (mild cellulitis; see below). Demographic characteristics of the participants are presented in Table 1. Of note, missing data were infrequent.
No serious adverse events or adverse events of special interest were reported, and vaccination pause rules were not implemented. As noted above, one participant did not receive a second vaccination owing to an unsolicited adverse event, mild cellulitis, that was associated with infection after an intravenous cannula placement to address an unrelated mild adverse event that occurred during the second week of follow-up. Second vaccination was withheld because the participant was still recovering and receiving antibiotics. This participant remains in the trial.
The percentage of participants in each vaccine group (groups A, B, C, D, and E) with adverse events according to the maximum FDA toxicity grade (mild, moderate, or severe) during the 7 days after each vaccination is plotted for solicited local (Panel A) and systemic (Panel B) adverse events. There were no grade 4 (life-threatening) events. Participants who reported 0 events make up the remainder of the 100% calculation (not displayed). Excluded were the three sentinel participants in groups C (5 μg + Matrix-M1, 5 μg + Matrix-M1) and D (25 μg + Matrix-M1, 25 μg + Matrix-M1), who received the trial vaccine in an open-label manner (see Table S7 for complete safety data on all participants).
Overall reactogenicity was largely absent or mild, and second vaccinations were neither withheld nor delayed due to reactogenicity. After the first vaccination, local and systemic reactogenicity was absent or mild in the majority of participants (local: 100%, 96%, 89%, 84%, and 88% of participants in groups A, B, C, D, and E, respectively; systemic: 91%, 92%, 96%, 68%, and 89%) who were unaware of treatment assignment (Figure 2 and Table S7). Two participants (2%), one each in groups D and E, had severe adverse events (headache, fatigue, and malaise). Two participants, one each in groups A and E, had reactogenicity events (fatigue, malaise, and tenderness) that extended 2 days after day 7. After the second vaccination, local and systemic reactogenicity were absent or mild in the majority of participants in the five groups (local: 100%, 100%, 65%, 67%, and 100% of participants, respectively; systemic: 86%, 84%, 73%, 58%, and 96%) who were unaware of treatment assignment. One participant, in group D, had a severe local event (tenderness), and eight participants, one or two participants in each group, had severe systemic events; the most common severe systemic events were joint pain and fatigue. Only one participant, in group D, had fever (temperature, 38.1°C) after the second vaccination, on day 1 only. No adverse event extended beyond 7 days after the second vaccination. Of note, the mean duration of reactogenicity events was 2 days or less for both the first vaccination and second vaccination periods.
Laboratory abnormalities of grade 2 or higher occurred in 13 participants (10%): 9 after the first vaccination and 4 after the second vaccination (Table S8). Abnormal laboratory values were not associated with any clinical manifestations and showed no worsening with repeat vaccination. Six participants (5%; five women and one man) had grade 2 or higher transient reductions in hemoglobin from baseline, with no evidence of hemolysis or microcytic anemia and with resolution within 7 to 21 days. Of the six, two had an absolute hemoglobin value (grade 2) that resolved or stabilized during the testing period. Four participants (3%), including one who had received placebo, had elevated liver enzymes that were noted after the first vaccination and resolved within 7 to 14 days (i.e., before the second vaccination). Vital signs remained stable immediately after vaccination and at all visits.
Unsolicited adverse events (Table S9) were predominantly mild in severity (in 71%, 91%, 83%, 90%, and 82% of participants in groups A, B, C, D, and E, respectively) and were similarly distributed across the groups receiving adjuvanted and unadjuvanted vaccine. There were no reports of severe adverse events.
Shown are geometric mean anti-spike IgG enzyme-linked immunosorbent assay (ELISA) unit responses to recombinant severe acute respiratory syndrome coronavirus 2 (rSARS-CoV-2) protein antigens (Panel A) and wild-type SARS-CoV-2 microneutralization assay at an inhibitory concentration greater than 99% (MN IC>99%) titer responses (Panel B) at baseline (day 0), 3 weeks after the first vaccination (day 21), and 2 weeks after the second vaccination (day 35) for the placebo group (group A), the 25-μg unadjuvanted group (group B), the 5-μg and 25-μg adjuvanted groups (groups C and D, respectively), and the 25-μg adjuvanted and placebo group (group E). Diamonds and whisker endpoints represent geometric mean titer values and 95% confidence intervals, respectively. The Covid-19 human convalescent serum panel includes specimens from PCR-confirmed Covid-19 participants, obtained from Baylor College of Medicine (29 specimens for ELISA and 32 specimens for MN IC>99%), with geometric mean titer values according to Covid-19 severity. The severity of Covid-19 is indicated by the colors of the dots for hospitalized patients (including those in intensive care), symptomatic outpatients (with samples collected in the emergency department), and asymptomatic patients who had been exposed to Covid-19 (with samples collected during contact and exposure assessment). Mean values (in black) for human convalescent serum are depicted next to (and of same color as) the category of Covid-19 patients, with the overall mean shown above the scatter plot (in black). For each trial vaccine group, the mean at day 35 is depicted above the scatterplot.
ELISA anti-spike IgG geometric mean ELISA units (GMEUs) ranged from 105 to 116 at day 0. By day 21, responses had occurred for all adjuvanted regimens (1984, 2626, and 3317 GMEUs for groups C, D, and E, respectively), and geometric mean fold rises (GMFRs) exceeded those induced without adjuvant by a factor of at least 10 (Figure 3 and Table S10). Within 7 days after the second vaccination with adjuvant (day 28; groups C and D), GMEUs had further increased by a factor of 8 (to 15,319 and 20,429, respectively) over responses seen with the first vaccination, and within 14 days (day 35), responses had more than doubled yet again (to 63,160 and 47,521, respectively), achieving GMFRs that were approximately 100 times greater than those observed with rSARS-CoV-2 alone. A single vaccination with adjuvant achieved GMEU levels similar to those in asymptomatic (exposed) patients with Covid-19 (1661), and a second vaccination with adjuvant achieved GMEU levels that exceeded those in convalescent serum from symptomatic outpatients with Covid-19 (7420) by a factor of at least 6 and rose to levels similar to those in convalescent serum from patients hospitalized with Covid-19 (53,391). The responses in the two-dose 5-μg and 25-μg adjuvanted vaccine regimens were similar, a finding that highlights the role of adjuvant dose sparing.
Neutralizing antibodies were undetectable before vaccination and had patterns of response similar to those of anti-spike antibodies after vaccination with adjuvant (Figure 3 and Table S11). After the first vaccination (day 21), GMFRs were approximately 5 times greater with adjuvant (5.2, 6.3, and 5.9 for groups C, D, and E, respectively) than without adjuvant (1.1). By day 35, second vaccinations with adjuvant induced an increase more than 100 times greater (195 and 165 for groups C and D, respectively) than single vaccinations without adjuvant. When compared with convalescent serum, second vaccinations with adjuvant resulted in GMT levels approximately 4 times greater (3906 and 3305 for groups C and D, respectively) than those in symptomatic outpatients with Covid-19 (837) and approached the magnitude of levels observed in hospitalized patients with COVID-19 (7457).
At day 35, ELISA anti-spike IgG GMEUs and neutralizing antibodies induced by the two-dose 5-μg and 25-μg adjuvanted vaccine regimens were 4 to 6 times greater than the geometric mean convalescent serum measures (8344 and 983, respectively).
Shown are scatter plots of 100% wild-type neutralizing antibody responses and anti-spike IgG ELISA unit responses at 3 weeks after the first vaccination (day 21) and 2 weeks after the second vaccination (day 35) for the two-dose 25-μg unadjuvanted vaccine (group B; Panel A), the combined two-dose 5-μg and 25-μg adjuvanted vaccine (groups C and D, respectively; Panel B), and convalescent serum from patients with Covid-19 (Panel C). In Panel C, the severity of Covid-19 is indicated by the colors of the dots for hospitalized patients (including those in intensive care), symptomatic outpatients (with samples collected in the emergency department), and asymptomatic patients who had been exposed to Covid-19 (with samples collected during contact and exposure assessment).
A strong correlation was observed between neutralizing antibody titers and anti-spike IgG GMEUs with adjuvanted vaccine at day 35 (correlation, 0.95) (Figure 4), a finding that was not observed with unadjuvanted vaccine (correlation, 0.76) but was similar to that of convalescent serum (correlation, 0.96). Two-dose regimens of 5-μg and 25-μg rSARS-CoV-2 plus Matrix-M1 produced similar magnitudes of response, and every participant had seroconversion according to either assay measurement. Reverse cumulative-distribution curves for day 35 are presented in Figure S2.
Frequencies of antigen-specific CD4+ T cells producing T helper 1 (Th1) cytokines interferon-gamma (IFN-γ), tumor necrosis factor-alpha (TNF-α), and interleukin-2 and for T helper 2 (Th2) cytokines interleukin-5 and interleukin-13 indicated cytokines from four participants each in the placebo (group A), 25-μg unadjuvanted (group B), 5-μg adjuvanted (group C), and 25-μg adjuvanted (group D) groups at baseline (day 0) and 1 week after the second vaccination (day 28) after stimulation with the recombinant spike protein. “Any 2Th1” indicates CD4+ T cells that can produce two types of Th1 cytokines at the same time. “All 3 Th1” indicates CD4+ T cells that produce IFN-γ, TNF-α, and interleukin-2 simultaneously. “Both Th2” indicates CD4+ T cells that can produce Th2 cytokines interleukin-5 and interleukin-13 at the same time.
T-cell responses in 16 participants who were randomly selected from groups A through D, 4 participants per group, showed that adjuvanted regimens induced antigen-specific polyfunctional CD4+ T-cell responses that were reflected in IFN-γ, IL-2, and TNF-α production on spike protein stimulation. A strong bias toward this Th1 phenotype was noted; Th2 responses (as measured by IL-5 and IL-13 cytokines) were minimal (Figure 5).