Replicating a COVID-19 study in a national England database to assess the generalisability of research with regional electronic health record data

Introduction The replication of observational studies using electronic health record data is critical for the evidence base of epidemiology. We have previously performed a study using linked primary and secondary care data in a large, urbanised region (Greater Manchester Care Record, Greater Manchester, UK) to compare the hospitalization rates of patients with diabetes (type 1 or type 2) after contracting COVID-19 with matched controls. Methods In this study we repeated the analysis using a national database covering the whole of England, UK (NHS England's Secure Data Environment service for England, accessed via the BHF Data Science Centre's CVD-COVID-UK/COVID-IMPACT Consortium). Results We found that many of the effect sizes did not show a statistically significant difference. Where effect sizes were statistically significant in the regional study, then they remained significant in the national study and the effect size was the same direction and of similar magnitude. Conclusion There is some evidence that the findings from studies in smaller regional datasets can be extrapolated to a larger, national setting. However, there were some significant differences and therefore replication studies remain an essential part of healthcare research.