IntroductionAt the start of the COVID-19 pandemic there was an urgent need to identify individuals at highest risk of severe outcomes, such as hospitalisation and death following infection. The QCOVID risk prediction algorithms emerged as key tools in facilitating this which were further developed during the second wave of the COVID-19 pandemic to identify groups of people at highest risk of severe COVID-19 related outcomes following one or two doses of vaccine.ObjectivesTo externally validate the QCOVID3 algorithm based on primary and secondary care records for Wales, UK.MethodsWe conducted an observational, prospective cohort based on electronic health care records for 1.66m vaccinated adults living in Wales on 8thDecember 2020, with follow-up until 15thJune 2021. Follow-up started from day 14 post vaccination to allow the full effect of the vaccine.ResultsThe scores produced by the QCOVID3 risk algorithm showed high levels of discrimination for both COVID-19 related deaths and hospital admissions and good calibration (Harrell C statistic: ≥ 0.828).ConclusionThis validation of the updated QCOVID3 risk algorithms in the adult vaccinated Welsh population has shown that the algorithms are valid for use in the Welsh population, and applicable on a population independent of the original study, which has not been previously reported. This study provides further evidence that the QCOVID algorithms can help inform public health risk management on the ongoing surveillance and intervention to manage COVID-19 related risks.