The interim results of a phase 1–2a clinical trial of the Ad26.COV2.S vaccine, a recombinant, replication-incompetent adenovirus serotype 26 (Ad26) vector encoding a full-length and stabilized SARS-CoV-2 spike protein, were reported. The trial was conducted in healthy adults aged 18–55 years and 65 years or older, with participants randomly assigned to receive either a low dose (5×10^10 viral particles) or a high dose (1×10^11 viral particles) of the vaccine in a single-dose or two-dose schedule. The primary endpoints were safety and reactogenicity. After the first dose, the most common solicited adverse events were fatigue, headache, myalgia, and injection-site pain, with fever being the most frequent systemic adverse event. Neutralizing-antibody titers against wild-type virus were detected in over 90% of participants by day 29 after the first dose, with titers stabilizing by day 57. A second dose further increased titers by a factor of 2.6 to 2.9. CD4+ T-cell responses were detected in 76–83% of participants in the younger cohort and 60–67% in the older cohort, with a skewing towards type 1 helper T cells. CD8+ T-cell responses were robust but lower in the older cohort. The interim analysis supports further development of the Ad26.COV2.S vaccine candidate.The interim results of a phase 1–2a clinical trial of the Ad26.COV2.S vaccine, a recombinant, replication-incompetent adenovirus serotype 26 (Ad26) vector encoding a full-length and stabilized SARS-CoV-2 spike protein, were reported. The trial was conducted in healthy adults aged 18–55 years and 65 years or older, with participants randomly assigned to receive either a low dose (5×10^10 viral particles) or a high dose (1×10^11 viral particles) of the vaccine in a single-dose or two-dose schedule. The primary endpoints were safety and reactogenicity. After the first dose, the most common solicited adverse events were fatigue, headache, myalgia, and injection-site pain, with fever being the most frequent systemic adverse event. Neutralizing-antibody titers against wild-type virus were detected in over 90% of participants by day 29 after the first dose, with titers stabilizing by day 57. A second dose further increased titers by a factor of 2.6 to 2.9. CD4+ T-cell responses were detected in 76–83% of participants in the younger cohort and 60–67% in the older cohort, with a skewing towards type 1 helper T cells. CD8+ T-cell responses were robust but lower in the older cohort. The interim analysis supports further development of the Ad26.COV2.S vaccine candidate.