This vaccine was tested on healthy adults aged 18 years or older who were infection free and were negative for previous SARS-CoV-2. This study provides evidence for the immunogenicity and safety of the Ad5-vectored COVID-19 vaccine in a larger population. Results suggest that a single-dose immunization schedule of Ad5-vectored COVID-19 vaccine is an appropriate treatment schedule for healthy adults. Older members of the population (ie, aged ≥55 years), showed to have significantly lower immune response, but higher tolerability to the vaccine, which indicates that an additional dose might be required to induce a better immune response in the older population. The primary objectives were to evaluate immunogenicity of the Ad5-vectored COVID-19 vaccine, and to determine a vaccine dose for a efficacy study. 603 volunteers were recruited and screened for eligibility between April 11-16, 2020.  Baseline characteristics of the participants and the pre-existing Ad5 neutralizing antibody were similar across the treatment groups. Among the 508 eligible participants who consented to participate, 266 (52%) had high pre-existing immunity and 242 (48%) had low pre-existing immunity to the Ad5 vector. The interpretation of the results leads to the conclusion that the Ad5- vectored COVID-19 vaccine is safe and induces a significant immune response for the majority of recipients after a single dose.