This study combines a mathematical model of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) transmission with multiple datasets to estimate the early dynamics of the infection in Wuhan, China, from December 2019 to February 2020. The model incorporates delays in symptom onset and reporting, and accounts for uncertainty in case observation. The results show that the median daily reproduction number (R) in Wuhan declined from 2.35 to 1.05 over a two-week period following the introduction of travel restrictions on January 23, 2020. Based on these estimates, the study assesses the potential for sustained human-to-human transmission in locations outside Wuhan if cases were introduced. The findings suggest that a single introduction of SARS-CoV-2 with SARS-like or MERS-like individual-level variation in transmission would have a 17% to 25% probability of causing a large outbreak in a new location. Once four or more infections are introduced, there is an over 50% chance that an outbreak will occur. The study highlights the importance of rapid case identification, isolation, and other control measures to reduce the risk of onward transmission chains.This study combines a mathematical model of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) transmission with multiple datasets to estimate the early dynamics of the infection in Wuhan, China, from December 2019 to February 2020. The model incorporates delays in symptom onset and reporting, and accounts for uncertainty in case observation. The results show that the median daily reproduction number (R) in Wuhan declined from 2.35 to 1.05 over a two-week period following the introduction of travel restrictions on January 23, 2020. Based on these estimates, the study assesses the potential for sustained human-to-human transmission in locations outside Wuhan if cases were introduced. The findings suggest that a single introduction of SARS-CoV-2 with SARS-like or MERS-like individual-level variation in transmission would have a 17% to 25% probability of causing a large outbreak in a new location. Once four or more infections are introduced, there is an over 50% chance that an outbreak will occur. The study highlights the importance of rapid case identification, isolation, and other control measures to reduce the risk of onward transmission chains.