The study reports on the safety, tolerability, and immunogenicity of the BNT162b1 vaccine, a lipid nanoparticle-formulated nucleoside-modified mRNA encoding the receptor binding domain (RBD) of the SARS-CoV-2 spike protein. The vaccine was administered in a dose-escalation/de-escalation setting to 60 healthy adults aged 18-55 years in Germany. Two doses of 1-50 μg of BNT162b1 elicited robust CD4+ and CD8+ T cell responses and strong antibody responses, with RBD-binding IgG concentrations significantly higher than those in serum from individuals who had recovered from COVID-19. Geometric mean titres of SARS-CoV-2 serum-neutralizing antibodies on day 43 were 0.7-fold to 3.5-fold higher than in convalescent serum samples. Immune sera broadly neutralized pseudoviruses with diverse SARS-CoV-2 spike variants. Most participants had T helper type 1 (T_H1)-skewed T cell immune responses with RBD-specific CD8+ and CD4+ T cell expansion. Interferon-γ was produced by a large fraction of RBD-specific CD8+ and CD4+ T cells. The robust RBD-specific antibody, T cell, and favorable cytokine responses induced by the BNT162b1 mRNA vaccine suggest that it has the potential to protect against COVID-19 through multiple beneficial mechanisms.The study reports on the safety, tolerability, and immunogenicity of the BNT162b1 vaccine, a lipid nanoparticle-formulated nucleoside-modified mRNA encoding the receptor binding domain (RBD) of the SARS-CoV-2 spike protein. The vaccine was administered in a dose-escalation/de-escalation setting to 60 healthy adults aged 18-55 years in Germany. Two doses of 1-50 μg of BNT162b1 elicited robust CD4+ and CD8+ T cell responses and strong antibody responses, with RBD-binding IgG concentrations significantly higher than those in serum from individuals who had recovered from COVID-19. Geometric mean titres of SARS-CoV-2 serum-neutralizing antibodies on day 43 were 0.7-fold to 3.5-fold higher than in convalescent serum samples. Immune sera broadly neutralized pseudoviruses with diverse SARS-CoV-2 spike variants. Most participants had T helper type 1 (T_H1)-skewed T cell immune responses with RBD-specific CD8+ and CD4+ T cell expansion. Interferon-γ was produced by a large fraction of RBD-specific CD8+ and CD4+ T cells. The robust RBD-specific antibody, T cell, and favorable cytokine responses induced by the BNT162b1 mRNA vaccine suggest that it has the potential to protect against COVID-19 through multiple beneficial mechanisms.