Due to the similarities with the COVID–19 pandemic, there has been a renewed interest in the 1918–1919 influenza pandemic, which represents the most severe pandemic of the 20th century with an estimated total death toll ranging between 30 and 100 million. This rapidly growing literature in economics and economic history has devoted attention to contextual determinants of excess mortality in the pandemic; to the impact of the pandemic on economic growth, inequality, and a range of other outcomes; and to the impact of nonpharmaceutical interventions. Estimating the effects of the pandemic, or the effects of countermeasures, is challenging. There may not be much exogenous variation to go by, and the historical data sets available are typically small and often of questionable quality. Yet the 1918–1919 pandemic offers a unique opportunity to learn how large pandemics play out in the long run. The studies evaluating effects of the pandemic, or of policies enacted to combat it, typically rely on some version of difference-in-differences, or instrumental variables. The assumptions required for these designs to achieve identification of causal effects have rarely been systematically evaluated in this particular historical context. Using a purpose-built dataset covering the entire Swedish population, such an assessment is provided here. The empirical analysis indicates that the identifying assumptions used in previous work may indeed be satisfied. However, the results cast some doubt on the general external validity of previous findings as the analysis fails to replicate several results in the Swedish context. These disagreements highlight the need for additional studies in other populations and contexts which puts the spotlight on further digitization and linkage of historical datasets.