We tracked the effective reproduction number (R) of the predominant severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variant Omicron BF.7 in Beijing in November–December 2022 by fitting a transmission dynamic model parameterized with real-time mobility data to (i) the daily number of new symptomatic cases on 1–11 November (when China’s zero-COVID interventions were still strictly enforced) and (ii) the proportion of individuals who participated in online polls on 10–22 December and self-reported to have been test-positive since 1 November. After China’s announcement of 20 measures to transition from zero-COVID, we estimated that R increased to 3.44 (95% credible interval (CrI): 2.82–4.14) on 18 November and the infection incidence peaked on 11 December. We estimated that the cumulative infection attack rate (IAR; that is, proportion of the population infected since 1 November) in Beijing was 75.7% (95% CrI: 60.7–84.4) on 22 December 2022 and 92.3% (95% CrI: 91.4–93.1) on 31 January 2023. Surveillance programs should be rapidly set up to monitor the evolving epidemiology and evolution of SARS-CoV-2 across China.