We propose a prognostic dynamic risk stratification for 48-hour in-hospital mortality in patients with COVID-19, using demographics and routinely-collected observations and laboratory tests: age, Clinical Frailty Scale score, heart rate, respiratory rate, SpO2/FiO2 ratio, white cell count, acidosis (pH < 7.35) and Interleukin-6. We train and validate the model using data from a UK teaching hospital, adopting a landmarking approach that accounts for competing risks and informative missingness. Internal validation of the model on the first wave of patients presenting between March 1 and September 12, 2020 achieves an AUROC of 0.90 (95% CI 0.87-0.93). Temporal validation on patients presenting between September 13, 2020 and January 1, 2021 gives an AUROC of 0.91 (95% CI 0.88-0.95). The resulting mortality stratification tool has the potential to provide physicians with an assessment of a patient's evolving prognosis throughout the course of active hospital treatment.