Phase 2 evaluation of two GEO-CM04S1 dose levels shows comparable safety and broad immunogenicity
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Updated: Sep 16, 2026
Determination of Vaccine Immunogenicity Using Bovine Monocyte-Derived Dendritic Cells
Published on: May 19, 2023
Sandra Ortega-Francisco1, Maria Teresa Prudente de Aquino2, Flavia Chiuppesi3
1Department of Hematology and HCT, Beckman Research Institute, City of Hope, Duarte, CA 91010, USA; Instituto de Biotecnologia, Universidad Nacional Autonoma de Mexico, Cuernavaca 62210, Mexico.
Background:
GEO-CM04S1 is a synthetic modified vaccinia Ankara (sMVA)-based SARS-CoV-2 vaccine expressing Spike (S) and Nucleocapsid (N) antigens derived from the ancestral Wuhan-Hu-1 reference strain. This phase 2 study evaluated the safety and immunogenicity of two GEO-CM04S1 dose levels administered as a single booster in healthy adults previously vaccinated against COVID-19.
Methods:
In this randomized, double-blind, parallel-group trial, participants received a single intramuscular booster of GEO-CM04S1 at either 1 × 10⁷ PFU (DL1) or 1 × 10⁸ PFU (DL2). Safety was assessed through adverse events and clinical evaluations. Humoral and cellular immune responses were measured for up to one year using SARS-CoV-2-specific IgG, neutralizing antibody assays against D614G, Omicron BA.1, and XBB.1.5, cytokine secretion, and activation-induced marker (AIM) assays.
Results:
Sixty-three participants were enrolled (31 DL1; 32 DL2). GEO-CM04S1 was well tolerated at both dose levels, with no vaccine-related serious adverse events. Both doses elicited robust binding and neutralizing antibody responses, including cross-neutralization of Omicron variants, and durable Th1-biased cellular immunity. DL2 induced modestly higher anti-S and anti-RBD binding antibody responses at selected time points, whereas neutralizing antibody and T-cell responses were comparable between dose groups.
Conclusions:
GEO-CM04S1 demonstrated a favourable safety profile and induced durable, broad humoral and cellular immune responses as a booster. Comparable functional immune responses between dose levels support the lower dose as a dose-sparing strategy for next-generation COVID-19 vaccination.
