Safety and immunogenicity of an MF59-adjuvanted spike glycoprotein-clamp vaccine for SARS-CoV-2: a randomised, double-blind, placebo-controlled, phase 1 trial

Abstract

Background: Given the scale of the ongoing COVID-19 pandemic, the development of vaccines based on different platforms is essential, particularly in light of emerging viral variants, the absence of information on vaccine-induced immune durability, and potential paediatric use. We aimed to assess the safety and immunogenicity of an MF59-adjuvanted subunit vaccine for COVID-19 based on recombinant SARS-CoV-2 spike glycoprotein stabilised in a pre-fusion conformation by a novel molecular clamp (spike glycoprotein-clamp [sclamp]).

Methods: We did a phase 1, double-blind, placebo-controlled, block-randomised trial of the sclamp subunit vaccine in a single clinical trial site in Brisbane, QLD, Australia. Healthy adults (aged ≥18 to ≤55 years) who had tested negative for SARS-CoV-2, reported no close contact with anyone with active or previous SARS-CoV-2 infection, and tested negative for pre-existing SARS-CoV-2 immunity were included. Participants were randomly assigned to one of five treatment groups and received two doses via intramuscular injection 28 days apart of either placebo, sclamp vaccine at 5 μg, 15 μg, or 45 μg, or one dose of sclamp vaccine at 45 μg followed by placebo. Participants and study personnel, except the dose administration personnel, were masked to treatment. The primary safety endpoints included solicited local and systemic adverse events in the 7 days after each dose and unsolicited adverse events up to 12 months after dosing. Here, data are reported up until day 57. Primary immunogenicity endpoints were antigen-specific IgG ELISA and SARS-CoV-2 microneutralisation assays assessed at 28 days after each dose. The study is ongoing and registered with ClinicalTrials.gov, NCT04495933.

Findings: Between June 23, 2020, and Aug 17, 2020, of 314 healthy volunteers screened, 120 were randomly assigned (n=24 per group), and 114 (95%) completed the study up to day 57 (mean age 32·5 years [SD 10·4], 65 [54%] male, 55 [46%] female). Severe solicited reactions were infrequent and occurred at similar rates in participants receiving placebo (two [8%] of 24) and the SARS-CoV-2 sclamp vaccine at any dose (three [3%] of 96). Both solicited reactions and unsolicited adverse events occurred at a similar frequency in participants receiving placebo and the SARS-CoV-2 sclamp vaccine. Solicited reactions occurred in 19 (79%) of 24 participants receiving placebo and 86 (90%) of 96 receiving the SARS-CoV-2 sclamp vaccine at any dose. Unsolicited adverse events occurred in seven (29%) of 24 participants receiving placebo and 35 (36%) of 96 participants receiving the SARS-CoV-2 sclamp vaccine at any dose. Vaccination with SARS-CoV-2 sclamp elicited a similar antigen-specific response irrespective of dose: 4 weeks after the initial dose (day 29) with 5 μg dose (geometric mean titre [GMT] 6400, 95% CI 3683-11 122), with 15 μg dose (7492, 4959-11 319), and the two 45 μg dose cohorts (8770, 5526-13 920 in the two-dose 45 μg cohort; 8793, 5570-13 881 in the single-dose 45 μg cohort); 4 weeks after the second dose (day 57) with two 5 μg doses (102 400, 64 857-161 676), with two 15 μg doses (74 725, 51 300-108 847), with two 45 μg doses (79 586, 55 430-114 268), only a single 45 μg dose (4795, 2858-8043). At day 57, 67 (99%) of 68 participants who received two doses of sclamp vaccine at any concentration produced a neutralising immune response, compared with six (25%) of 24 who received a single 45 μg dose and none of 22 who received placebo. Participants receiving two doses of sclamp vaccine elicited similar neutralisation titres, irrespective of dose: two 5 μg doses (GMT 228, 95% CI 146-356), two 15 μg doses (230, 170-312), and two 45 μg doses (239, 187-307).

Interpretation: This first-in-human trial shows that a subunit vaccine comprising mammalian cell culture-derived, MF59-adjuvanted, molecular clamp-stabilised recombinant spike protein elicits strong immune responses with a promising safety profile. However, the glycoprotein 41 peptide present in the clamp created HIV diagnostic assay interference, a possible barrier to widespread use highlighting the criticality of potential non-spike directed immunogenicity during vaccine development. Studies are ongoing with alternative molecular clamp trimerisation domains to ameliorate this response.

Funding: Coalition for Epidemic Preparedness Innovations, National Health and Medical Research Council, Queensland Government, and further philanthropic sources listed in the acknowledgments.

Figure 1

Trial profile *Includes one sentinel participant whose immediate post-dose safety data were reviewed by the safety review committee before proceeding with dosing of the rest of the cohort. † Cumulative safety data from the first 7 days after the first dose of each cohort were reviewed by the safety review committee before proceeding to the next higher-dose cohort.

Figure 2

Adverse events Percentage of (A) solicited local adverse events and (B) solicited systemic adverse events within 7 days of first and second dose. Data for the first dose of treatment C (two doses of 45 μg SARS-CoV-2 sclamp) and D (one dose of 45 μg SARS-CoV-2 sclamp, followed by one dose of placebo) were pooled (n=48), whereas data for the second dose are shown separately. Sclamp=spike glycoprotein-clamp vaccine. *Solicited adverse events after saline dosing in the single 45 μg dose cohort.

Figure 3

Serum antibody response to vaccine Circles represent geometric mean and error bars represent the geometric mean SD at each timepoint. (A) Anti-SARS-CoV-2 sclamp IgG ELISA responses in trial participants. The dotted line represents the limit of detection. Adjusted p values compare treatment versus placebo (two-way ANOVA Dunnett's multiple comparison test). (B) Live SARS-CoV-2 microneutralisation in trial participants and from serum of convalescent patients with COVID-19 infected with SARS-CoV-2 during the first outbreak of the pandemic in Victoria, Australia. Dashed line represents the geometric mean MN₅₀ of convalescent serum of patients with COVID-19 and the dotted line represents the limit of detection. Adjusted p value versus placebo (two-way ANOVA Dunnett's multiple comparison test). (C) Surrogate virus neutralisation assay results in trial participants at each timepoint. The dotted line represents 20% inhibition below which readings are considered negative. Adjusted p value versus placebo (two-way ANOVA Dunnett's multiple comparison test). (D) SARS-CoV-2 pseudovirus neutralisation in trial participants at day 57. The dotted line represents the limit of detection. Adjusted p value versus placebo (one-way ANOVA Dunn's multiple comparison test). (E) Anti-SARS-CoV-2 sclamp IgG ELISA responses in trial participants; titres of antibodies specific for the SARS-CoV-2 sclamp and Nipah Fclamp antigens at day 57 and the percentage of the anti-SARS-CoV-2 sclamp IgG ELISA response that was specific to the clamp trimerisation domain are shown. The dotted line represents the limit of detection. Adjusted p value of sclamp versus clamp (two-way ANOVA Sidak's multiple comparison test). (F) Reactivity of trial participants' serum at day 57. For comparison of reactivity, samples from 50 patients with HIV are shown (pink). ACE2=angiotensin-converting enzyme 2. MN₅₀=50% microneutralisation. Sclamp=spike glycoprotein-clamp vaccine. RBD=receptor binding domain. psNT50=50% pseudovirus neutralisation titre. *p<0·0001.

Figure 4

SARS-CoV-2 spike-specific cytokine expression by CD4⁺T cells, antibody-secreting cells, and Tfh1 responses in peripheral blood Summation analysis of SARS-CoV-2 spike-specific Th1 cytokine (IFNγ, TNF, and IL-2) expression by CD4⁺ T cells at each timepoint (A). SARS-CoV-2 spike-specific Th1 and Th2 cytokine expression by CD4⁺ T cells at day 43 (B). Fold-change in the frequency of CD3⁻D19⁺CD27^hiCD38^hi antibody-secreting cells (C), activated ICOS⁺PD-1⁺CXCR5⁺CXCR3⁺CCR6⁻CD4⁺ circulating Tfh1 cells (D), and activated ICOS⁺CD38⁺CCR5⁻CD4⁺ Th1 cells (E), at day 8 and 36. Bars and lines and error bars indicate the median (IQR). Statistical significance between different groups was determined using a fitted mixed-model two-way ANOVA. Tfh1=type 1 T follicular helper cells. Sclamp=spike glycoprotein-clamp vaccine. Th=T-helper. TNF=tumour necrosis factor. ICOS=inducible T cell costimulator. *p<0·0001.