In the phase 2 trial of the Ad5-nCoV vaccine, no significant difference was found between the high-dose and low-dose groups in specific antibody production. However, positive T cell responses were measured in 90% and 88% of the high and low-dose groups, respectively. Preexisting immunity and older age negatively impacted the responses to vaccination. Most adverse reactions were mild to moderate and resolved in less than two days, although worse adverse reactions were more frequently reported in the high-dose group. The low-dose group demonstrated a good safety profile, supporting its progression to phase 3 testing.