Digital technologies in the public-healthresponse to COVID-19

C , was declared a pandemic bythe World Health Organization (WHO) on 11 March 2020, less than 3 months after cases were first detected. With now over 9.8 million confirmed cases and more than 495,000 deaths3 recorded worldwide, there are grave concerns about the global health, societal and economic effects of this virus, particularly on vulnerable and disadvantaged populations, and in low- and middle-income countries with fragile health systems4,5 . At the time of this writing, 7.1 billion people live in countries that have had substantial travel and social restrictions6 . As with the control of outbreaks and pandemics before it, controlling the COVID-19 pandemic rests on the detection and containment of clusters of infection and the interruption of community transmission to mitigate the impact on human health. During the plague outbreak that affected 14th-century Europe, isolation of affected communities and restriction of population movement were used to avoid further spread7 . These public-health measures for outbreak response remain relevant today, including surveillance, rapid case identification, interruption of community transmission and strong public communication. Monitoring how these measures are implemented and their impact on incidence and mortality is essential. All countries are required by the International Health Regulations (2005)8 to have core capacity to ensure national preparedness for infectious hazards that have the potential to spread internationally. Research and development of new methods and technologies to strengthen these core capacities often occurs during outbreaks, when innovation is an absolute necessity9 . During the outbreak of severe acute respiratory syndrome in 2003, Hong Kong identi- fied clusters of disease through the use of electronic data systems10 . During the Ebola outbreaks in West Africa in 2014–2016, mobile phone data were used to model travel patterns11, and hand-held sequencing devices permitted more-effective contact tracing and a better understanding of the dynamics of the outbreaks12. Similarly, digital technologies also have been deployed in the COVID-19 pandemic13,14 (Table 1) to strengthen each of the four public-health measures noted above. The digital revolution has transformed many aspects of life. As of 2019, 67% of the global population had subscribed to mobile devices, of which 65% were smartphones—with the fastest growth in Sub-Saharan Africa15 . In 2019, 204 billion apps were downloaded16, and as of January 2020, 3.8 billion people actively used social media17 . Here we critically review how digital technologies are being harnessed for the public-health response to COVID-19 worldwide (Fig. 1). We discuss the breadth of innovations and their respective limitations. This systems-level approach is needed to inform how digital strategies can be incorporated into COVID-19-control strategies, and to help prepare for future epidemics. Digital epidemiological surveillance A core public-health function of outbreak management is understanding infection transmission in time, place and person, and identifying risk factors for the disease to