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RESEARCH ◥ 0 to 9 and less than 10% attributable to early RESEARCH ARTICLE SUMMARY adolescents and teenagers aged 10 to 19. Thus, adults aged 20 to 49 continue to be the only CORONAVIRUS age groups that contribute disproportionately to COVID-19 spread relative to their size in the Age groups that sustain resurging COVID-19 population (see figure panel B). However, be- cause children and teenagers seed infections epidemics in the United States among adults who are more transmission ef- ficient, we estimate that overall, school opening Mélodie Monod*, Alexandra Blenkinsop*, Xiaoyue Xi*, Daniel Hebert*, Sivan Bershan*, Simon Tietze*, is indirectly associated with a 26% increase in Marc Baguelin, Valerie C. Bradley, Yu Chen, Helen Coupland, Sarah Filippi, Jonathan Ish-Horowicz, SARS-CoV-2 transmission. Martin McManus, Thomas Mellan, Axel Gandy, Michael Hutchinson, H. Juliette T. Unwin, Sabine L. van Elsland, Michaela A. C. Vollmer, Sebastian Weber, Harrison Zhu, Anne Bezancon, CONCLUSION: We show that considering trans- Neil M. Ferguson, Swapnil Mishra, Seth Flaxman†, Samir Bhatt†, mission through the lens of contact patterns is Oliver Ratmann*†, on behalf of the Imperial College COVID-19 Response Team fundamental to understanding which popula- tion groups are driving disease transmission. Over time, the share of age groups among INTRODUCTION: After initial declines, in mid- lation groups should be informed by the reported deaths has been markedly constant, 2020, a sustained resurgence in the transmis- demographics currently driving and sustaining and the data provide no evidence that trans- sion of novel coronavirus disease (COVID-19) transmission. To develop the tools to answer mission shifted to younger age groups before Downloaded from occurred in the United States. Throughout such questions, we introduce a new framework school reopening, and no evidence that young the US epidemic, considerable heterogeneity that links mobility to mortality through age- adults aged 20 to 34 were the primary source existed among states, both in terms of overall specific contact patterns and then use this rich of resurgent epidemics since the summer of mortality and infection, but also in the types relationship to reconstruct accurate transmis- 2020. Our key conclusion is that in locations and stringency of nonpharmaceutical inter- sion dynamics (see figure panel A). where novel, highly transmissible SARS-CoV-2 ventions. Despite these stark differences among lineages have not yet become established, states, little is known about the relationship be- RESULTS: We find that as of 29 October 2020, additional interventions among adults aged http://science.sciencemag.org/ tween interventions, contact patterns, and infec- adults aged 20 to 34 and 35 to 49 are the 20 to 49, such as mass vaccination with tions, or how this varies by age and demographics. only age groups that have sustained SARS- transmission-blocking vaccines, could bring A useful tool for studying these dynamics is in- CoV-2 transmission with reproduction num- resurgent COVID-19 epidemics under control dividual, age-specific mobility data. In this study, bers (transmission rates) consistently above and avert deaths.▪ we use detailed mobile-phone data from more one. The high reproduction numbers from than 10 million individuals and establish a mech- adults are linked both to rebounding mobil- The list of authors and their affiliations is available in the full anistic relationship between individual contact ity over the summer and elevated transmis- article online. patterns and COVID-19 mortality data. sion risks per venue visit among adults aged *These authors contributed equally to this work. †Corresponding author. Email: [email protected]. 20 to 49. Before school reopening, we estimate uk (O.R.); [email protected] (S.B.); s.flaxman@ imperial.ac.uk (S.F.) RATIONALE: As the pandemic progresses, dis- that 75 of 100 COVID-19 infections originated on March 26, 2021 ease control responses are becoming increas- from adults aged 20 to 49, and the share of This is an open-access article distributed under the terms of the Creative Commons Attribution license (https://creative- ingly nuanced and targeted. Understanding young adults aged 20 to 34 among COVID-19 commons.org/licenses/by/4.0/), which permits unrestricted fine-scale patterns of how individuals interact infections was highly variable geographically. use, distribution, and reproduction in any medium, provided with each other is essential to mounting an After school reopening, we reconstruct rela- the original work is properly cited. Cite this article as M. Monod et al., Science 371, eabe8372 efficient public health control program. For tively modest shifts in the age-specific sources (2021). DOI: 10.1126/science.abe8372 example, the choice of closing workplaces, of resurgent COVID-19 toward younger in- closing schools, limiting hospitality sectors, dividuals, with less than 5% of SARS-CoV-2 READ THE FULL ARTICLE AT or prioritizing vaccination to certain popu- transmissions attributable to children aged https://doi.org/10.1126/science.abe8372 A Baseline contacts POLYMOD contact B Estimated sources of SARS-CoV-2 infections, Time varying survey pre-pandemic United States average October 2020 Inferred person-person predictors ages 20+ Age-specific cell phone 50% contacts by 5 year age derived mobility trends bands over time Contacts during 40 NPIs ages 0-19 Contact surveys Per state during pandemic 30 per day 20 Natural disease Informed by contact Modeled SARS-COV-2 parameters tracing data and large- 10 infection dynamics scale seroprevalence Percent of population Heterogeneity in human 0 Per state per day surveys behavior and disease 0-9 10-19 20-34 35-49 50-64 65-79 80+ transmission modeled with Age group Expected deaths Age-specific random effects in space, time, mortality data Contribution to Share in and by age Per state per day SARS-CoV-2 infections population Model developed to estimate the contribution of age groups to resurgent COVID-19 epidemics in the United States. (A) Model overview. (B) Estimated contribution of age groups to SARS-CoV-2 transmission in October. Monod et al., Science 371, 1336 (2021) 26 March 2021 1of1 RESEARCH ◥ though such data are essential to better RESEARCH ARTICLE understand the engines of COVID-19 trans- mission (10). CORONAVIRUS Cell-phone data suggest similar rebounds in Age groups that sustain resurging COVID-19 mobility across age groups We compiled a national-level, aggregate mo- epidemics in the United States bility data set using cell phone data from >10 million individuals with Foursquare’s Mélodie Monod1*, Alexandra Blenkinsop1*, Xiaoyue Xi1*, Daniel Hebert2*, Sivan Bershan3*, location technology, Pilgrim (11), which lever- Simon Tietze3*, Marc Baguelin4, Valerie C. Bradley5, Yu Chen1, Helen Coupland4, Sarah Filippi1, ages a wide variety of mobile device signals Jonathan Ish-Horowicz1, Martin McManus1, Thomas Mellan4, Axel Gandy1, Michael Hutchinson5, to pinpoint the time, duration, and location H. Juliette T. Unwin4, Sabine L. van Elsland4, Michaela A. C. Vollmer4, Sebastian Weber6, of user visits to locations such as shops, parks, Harrison Zhu1, Anne Bezancon3, Neil M. Ferguson4, Swapnil Mishra4, Seth Flaxman1†, or universities. Unlike the population-level Samir Bhatt4,7†, Oliver Ratmann1*†, on behalf of the Imperial College COVID-19 Response Team mobility trends published by Google from cell phone geolocation data (12), the data are After initial declines, in mid-2020 a resurgence in transmission of novel coronavirus disease (COVID-19) disaggregated by age. User venue visits were occurred in the United States and Europe. As efforts to control COVID-19 disease are reintensified, aggregated and projected to estimate, for each understanding the age demographics driving transmission and how these affect the loosening of state, and two metropolitan areas, daily per- interventions is crucial. We analyze aggregated, age-specific mobility trends from more than 10 million centage changes in venue visits for individuals Downloaded from individuals in the United States and link these mechanistically to age-specific COVID-19 mortality data. aged 18 to 24, 25 to 34, 35 to 44, 45 to 54, 55 We estimate that as of October 2020, individuals aged 20 to 49 are the only age groups sustaining to 64, and 65+ years relative to the baseline resurgent SARS-CoV-2 transmission with reproduction numbers well above one and that at least 65 of period 3 to 9 February 2020 (figs. S1 and S2 100 COVID-19 infections originate from individuals aged 20 to 49 in the United States. Targeting and supplementary materials). interventions—including transmission-blocking vaccines—to adults aged 20 to 49 is an important Across the US as a whole, the mobility consideration in halting resurgent epidemics and preventing COVID-19–attributable deaths. trends indicate substantial initial declines in venue visits, followed by a subsequent re- http://science.sciencemag.org/ bound for all age groups (Fig. 1A and fig. S1). ollowing worldwide spread of the novel ceutical interventions, changing contact in- During the initial phase of epidemic spread, severe acute respiratory syndrome coro- tensities, age, and other factors have inter- trends declined most strongly among indi- navirus 2 (SARS-CoV-2), the implemen- acted and led to resurgent disease spread. viduals aged 18 to 24 years across almost all F tation of large-scale nonpharmaceutical We test previous claims that resurgent COVID- states and metropolitan areas and subse- interventions has led to sustained declines 19 is a result of increased spread from young quently tended to increase most strongly in the number of reported SARS-CoV-2 infec- adults, identify the population age groups among individuals aged 18 to 24 in the majority tions and deaths from COVID-19 (1, 2). How- driving SARS-CoV-2 spread across the US of states and metropolitan areas (fig. S3), con- ever, since mid-June 2020, the daily number through 29 October 2020, and quantify changes sistent with reopening policies for restaurants, of reported COVID-19 cases has been resurg- in transmission dynamics since schools night clubs, and other venues (10, 13, 14).