Phase 1/2 clinical trials of the ChAdOx1 nCoV-19 (AZD1222) vaccine demonstrated an acceptable safety profile and robust immunogenicity. Mild systemic and local reactions, such as fatigue, fever, and headache, were common but peaked on the first day and were mitigated by prophylactic paracetamol. Immunologically, the vaccine induced spike-specific effector T-cell responses that peaked by week two, and serological antibodies that peaked by week four. A second dose further enhanced neutralizing antibody titers, although it did not significantly increase cellular immune responses.