evidence: Conclusions The study provides reassuring data on low rates of AEs after COVID-19 vaccination. Differences in reactogenicity-type AE profiles between ChAdOx1 and BNT162b2 and between first and second doses of these vaccines were observed. [...] VAC4COVID (SARS-CoV-2 Vaccination for COVID-19 Disease Safety Study) is an independent academic UK-wide web-based study designed to collect patient-reported adverse medical events before and after vaccination. The study design also allows clinical validation of any serious or non-serious adverse events of special interest (AESIs).10 We present an interim analysis of AR data collected between 2 February 2021 and 5 October 2021 in the VAC4COVID study. [...] The two most commonly used COVID-19 vaccines in the UK, ChAdOx1 and BNT162b2, are well tolerated. The majority of vaccine recipients in this study reported no adverse health events after vaccination, and most reported events were of mild or moderate severity. Furthermore, a high proportion of participants reported subjective improvements in overall health and well-being after vaccination. These data should be reassuring to people contemplating vaccination. However, there are differences between the reactogenicity-related symptom profiles of the vaccines that should be considered in planning future vaccination programmes. The ongoing VAC4COVID remote active surveillance system is designed to monitor vaccine safety and effectiveness by combining participant-reported data with linked health and administrative data; the flexible design would allow data collection in any country for which there is a suitable translated MedDRA dictionary. As well as providing comparative COVID-19 vaccine AE data, the system may also be adapted to monitor new non-COVID vaccines and other medications.