The main objective of this study was to determine the appropriate dose for this new vaccine with adenovirus type-5 vectors. The study was double-blind, randomized, with a placebo group, in a center in the city of Wuhan, China. The inclusion criteria consisted of adults> 18 years of age, with HIV negative and without previous respiratory disease due to COVID-19. They were divided into 3 groups, the first group consisted of a dose of 1x1011 viral particles per ml [n = 253], to the second group doses of 5x1010 viral particles per ml [n = 126] and the third group placebo [n = 126] . Participants, researchers, and staff were blinded. Immunogenicity endpoints were geometric mean titers (GMT) of receptor-binding domain (RBD) -specific ELISA antibody responses and neutralizing antibody responses on day 28. Endpoint for evaluation of the Safety was the incidence of adverse reactions within 14 days. The sample consisted of 603 volunteers, where 508 were eligible being 50% men, average age of 39.7, who were randomly assigned to one of the 3 groups. Both vaccine groups induced anti-SARS-CoV-2 antibody responses, with GMT of 19 • 5 (95% CI: 16 • 8–22 • 7) and 18 • 3 (14 • 4–23 • 3) in participants who received 1 × 1011 and 5 × 1010 viral particles, respectively. Adverse reactions were documented in 72% in the first group and 74% in the second group, with only 1% serious adverse reactions, concluding that the COVID-19 vaccine with Ad5 a vector (5 × 1010) is safe and generates a significant immune response after a single immunization. It is worth mentioning that all the participants are from the city of Wuhan, which could cause a variation of the real effectiveness.