Evaluating the safety, tolerability, and immunogenicity of a 24-valent pneumococcal conjugate vaccine (VAX-24) in healthy adults aged 18 to 64 years: a phase 1/2, double-masked, dose-finding, active-controlled, randomised clinical trial.

Background: Despite substantial reductions in pneumococcal disease with the availability of pneumococcal conjugate vaccines, a significant burden of pneumococcal disease remains due to the diversity of serotypes combined with serotype replacement. We developed a new vaccine candidate, VAX-24 (24-valent pneumococcal conjugate vaccine), using cell-free protein synthesis to produce a variant of cross-reactive material 197 (eCRM) as the carrier protein, increasing serotype coverage while minimising carrier suppression. The aim of this clinical trial was to assess the safety, tolerability, and immunogenicity of three different doses of VAX-24 compared to pneumococcal 20-valent conjugate vaccine (PCV20).
Methods: This was a phase 1/2, randomised, double-masked study of VAX-24 versus PCV20 conducted in the USA. Key inclusion criteria included being a male or female aged 18 to 64 years in good health; key exclusion criteria included previous history of pneumococcal disease, receipt of a licensed or investigational pneumococcal vaccine, or immunosuppressive therapy. Participants were randomly allocated in a 1:1:1:1 ratio by permuted block to receive one dose of VAX-24 (1·1 μg of each antigen, 2·2 μg of each antigen, or 2·2 μg of 17 antigens mixed with 4·4 μg of seven antigens), or PCV20. The safety population included all participants with safety data. The immunogenicity population was as per-treatment in phase 2. Primary outcome measures included solicited and unsolicited adverse events. Secondary outcomes included serotype-specific opsonophagocytic activity (OPA) geometric mean titres (GMT), and IgG geometric mean concentrations (GMC) were measured 1 month postvaccination. Traditional non-inferiority criteria included OPA geometric mean ratio (GMR), with a lower bound of the two sided 95% CI of greater than 0·5 for shared serotypes. This completed trial is registered at ClinicalTrials.gov, NCT05266456.
Findings: Safety profiles were comparable among the treatment groups, with 170 of 209 participants (81%, 95% CI 75·2-86·2) to 178 of 207 participants (86%, 80·5-90·4) reporting at least one solicited adverse event among the three VAX-24 groups. 24 of 207 participants (12%, 7·6-16·8) to 32 of 209 of participants (15%, 10·7-20·9) experiened an unsolicited treatment emergent adverse event within 1 month postvaccination. VAX-24 2·2 μg met traditional OPA GMR non-inferiority criteria for all 20 shared serotypes; 16 serotypes elicited GMR point estimates greater than 1·0, and four reached the lower bound of the two-sided 95% CI greater than 1·0.
Interpretation: VAX-24 had a safety profile similar to PCV20 at all doses, with the 2·2 μg dose showing increased serotype coverage with decreased carrier suppression.
Funding: Vaxcyte.
Competing Interests: Declaration of interests JW, MS, JS, PS, DJ, KN, TSM, SI, and JF are all employees of the trial sponsor. SB and MC are consultants for Vaxcyte. JW, MS, JF, DJ, T-SM, KN, PS, and JKS are employees of, and own stock or hold stock options, in Vaxcyte. MD and CF are investigators who worked on the study.
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