By late summer 2020, the resurgence of COVID-19 in the United States was largely driven by adults between the ages of 20 and 49, a new study finds. The results indicate that in locations where novel highly transmissible SARS-CoV-2 lineages have not yet established, additional interventions among adults of these ages could bring resurgent COVID-19 epidemics under control and avert deaths. Following initial declines in the number of reported SARS-CoV-2 infections and deaths - a result largely attributed to non-pharmaceutical interventions - a resurgence in transmission of COVID-19 occurred in the United States and Europe beginning in August 2020. Understanding the age demographics that drove this is crucial. For example, between August and October 2020, school closure mandates were lifted in many United States locations. Whether the resurgent epidemics of SARS-CoV-2 during 2020 can be explained by students going back to school has been an open question. To help address this, Mélodie Monod et al. used detailed, longitudinal, and age-specific population mobility and COVID-19 mortality data to estimate how non-pharmaceutical interventions, changing contact intensities, age, and other factors interplayed. Part of the authors' approach involved analyzing aggregated, age-specific mobility trends from more than 10 million individuals' cell phones. The mobile device signals could be used to pinpoint the time, duration, and location of user visits to U.S. locations such as shops, parks, or universities. The authors incorporated this mobility data into a Bayesian contact-and-infection model. As of their last observation week in October 2020, and following evaluations of reproduction numbers - which consider secondary infections one infected person generates - and data on transmission chains, their results suggest school reopening did not result in substantial increases in COVID-19 attributable deaths. Instead, data suggest that SARS-CoV-2 transmission was sustained primarily by people in age groups 20-49. "[A]dults aged 20-49 naturally have most contacts to other adults aged 20 and above" and these, the authors say, are both more susceptible and more mobile than younger age groups. They conclude: "This study provides evidence that the resurgent COVID-19 epidemics in the US in 2020 have been driven by adults aged 20-49, and in particular adults aged 35-49, before and after school reopening."