  The swift relaxation of the zero-COVID policy in December 2022 led to an
unprecedented surge in Omicron variant infections in China. With the suspension
of mandatory testing, tracking this epidemic outbreak was challenging because
infections were often underrepresented in survey and testing results, which
only involved partial populations. We used large-scale mobile phone data to
estimate daily infections in Beijing from November 2022 to January 2023. We
demonstrated that an individual's location records of mobile phone could be
used to infer his or her infectious status. Then, the derived status of
millions of individuals could be summed to reconstruct the citywide
spatiotemporal dynamics of infections. We found that the infection incidence
peaked on 21 December, and 80.1% of populations had been infected by 14 January
2023 in Beijing. Furthermore, infection dynamics exhibited significant
demographic and spatiotemporal disparities. Our work provides a ubiquitous and
high-coverage data source for monitoring epidemic outbreaks.
