RATIONALE: Follow-up of pre-licensure trial participants potentially informs a vaccine's long-term safety profile. DESIGN: This prospective study enrolled participants from the first clinical trials of ChAdOx1 nCoV-19 vaccine, many of whom subsequently received COVID-19 vaccines in the UK vaccination programme. Serious adverse events (SAEs) and adverse events of special interest (AESIs) were captured. AESI selection was based on Brighton Collaboration definitions. RESULTS: Between September 2021 and April 2022, 4470 participants enrolled (58% female, median age 50 years); 3845/4470 (86.0%) completed the study. Median follow-up was 11.77 months; total follow-up was 48,116 person-months. There were 174 SAEs and 71 AESIs (33 events recorded as both) in 195 participants, including 5 deaths. Immunology data were available for a small subset of participants; total IgG against trimeric SARS-CoV-2 spike protein tended to increase during the study. CONCLUSIONS: Although no emergent safety signals were detected, continued post-marketing surveillance remains imperative, particularly for neurological disorders. EUDRACT NUMBER: 2021-003382-36.