Informing how COVID-19 response plans may incorporate digital contact tracing, a model of COVID-19 spread within a simulated French population found that if about 20% of the population adopted a contact tracing app on their smartphones, an outbreak could be reduced by about 35%. If more than 30% of the population adopted the app, the epidemic could be suppressed to manageable levels. Jesús Moreno López and colleagues note that the effectiveness of digital contact tracing would depend on a given population's level of immunity to the virus; the intervention alone would be unable to suppress a COVID-19 epidemic where transmission - and especially asymptomatic transmission - remains high. While many countries have implemented lockdowns and rigorous social distancing measures when COVID-19 activity has crescendoed, these interventions are typically short-lived, with new waves emerging after restrictions are lifted. More sustainable strategies, such as digital contact tracing, are important for keeping outbreaks under control. However, scientists have debated the efficacy of digital contact tracing, since in many countries, a large portion of the population does not have smartphones - particularly the elderly. To investigate digital contact tracing's ability to mediate the spread of COVID-19, Moreno López et al. developed a model that simulates a synthetic French population based on census data from the National Institute of Statistics and Economic Studies (INSEE). The researchers used this synthetic population to explore the impact of digital contact tracing - as well as detection of COVID-19 cases, quarantines, and isolation of household contacts - under scenarios in which the virus was more or less transmissible based on the prevalence of face mask use and hand washing. They found that when the virus was highly transmissible, household isolation alone reduced COVID-19 cases by 27%, while pairing this strategy with digital contact tracing reduced COVID-19 cases by 35% when only 20% of the population adopted the app. Simulating increased rates of app adoption also led to further reductions in cases. Moreno López et al. note that additional information would be required to adapt this approach to a specific outbreak within a particular country.