Davis et al. used the Global Epidemic and Mobility (GLEAM) model, which has both stochastic (incorporating elements of randomness) and mechanistic (including defined principles about the biological and social mechanisms associated with viral infection and transmission) components to simulate virus spread on a global scale2. The model also captures biological aspects of SARS-CoV-2 transmission, clinical features (such as lethality for each age group analysed) and the timing of non-pharmaceutical containment measures, such as lockdowns. Methods that calibrate these models to both epidemiological and virus-sequencing data could improve their performance for 'now-casting' and forecasting viral spread, as well as for assessing the effects of policies in real time.