The lack of seasonal influenza epidemics has been an unexpected feature of the COVID-19 pandemic, illustrated by WHO sentinel surveillance data from temperate Southern Hemisphere countries demonstrating a near absence of influenza since early 2020.1 These observational data were consistent with the hypothesis that non-pharmaceutical interventions (NPIs) introduced to control the spread of COVID-19 could have dramatically reduced the burden of influenza and prevented seasonal epidemics. However, this finding did not shed light on the relative contribution of specific NPIs (such as widespread face mask use, school and workplace closures, physical distancing, travel restrictions, limits on gathering sizes, and border control measures) on reducing influenza activity, if the association was indeed causative.