A mathematical model was used to analyze the early transmission dynamics of SARS-CoV-2 in Wuhan, China, and assess the potential for sustained human-to-human transmission outside the region. The study combined a stochastic transmission model with four datasets from within and outside Wuhan, estimating how transmission varied between December 2019 and February 2020. The median daily reproduction number (Rd) in Wuhan declined from 2.35 (95% CI 1.15–4.77) one week before travel restrictions were introduced on January 23, 2020, to 1.05 (0.41–2.39) one week after. Based on these estimates, the study found that in locations with similar transmission potential to Wuhan in early January, once there are at least four independently introduced cases, there is a more than 50% chance the infection will establish within that population. The model accounted for delays in symptom onset and reporting, and incorporated uncertainty in case observation. It also considered the impact of international travel and the probability of cases being exported from Wuhan to other countries. The study found that the model reproduced the observed temporal trend of cases within Wuhan and internationally, and that the majority of cases were symptomatic. The model also estimated that 94.8% of the Wuhan population were still susceptible on January 31, 2020. The study highlights the importance of rapid case identification and isolation to reduce the chance of onward transmission. It also shows that combining multiple data sources is valuable for analyzing the transmission dynamics of COVID-19. The results suggest that transmission in Wuhan declined in late January 2020, coinciding with the introduction of travel control measures. The study also found that the estimated reproduction number and published estimates of individual-level variation in transmission for SARS-CoV and MERS-CoV indicate that a single case introduced to a new location may not necessarily lead to an outbreak, as high individual-level variation in transmission makes new chains of transmission more fragile. The study concludes that understanding the effectiveness of control measures in different settings is crucial for understanding the dynamics of the outbreak and the likelihood that transmission can eventually be contained or effectively mitigated.A mathematical model was used to analyze the early transmission dynamics of SARS-CoV-2 in Wuhan, China, and assess the potential for sustained human-to-human transmission outside the region. The study combined a stochastic transmission model with four datasets from within and outside Wuhan, estimating how transmission varied between December 2019 and February 2020. The median daily reproduction number (Rd) in Wuhan declined from 2.35 (95% CI 1.15–4.77) one week before travel restrictions were introduced on January 23, 2020, to 1.05 (0.41–2.39) one week after. Based on these estimates, the study found that in locations with similar transmission potential to Wuhan in early January, once there are at least four independently introduced cases, there is a more than 50% chance the infection will establish within that population. The model accounted for delays in symptom onset and reporting, and incorporated uncertainty in case observation. It also considered the impact of international travel and the probability of cases being exported from Wuhan to other countries. The study found that the model reproduced the observed temporal trend of cases within Wuhan and internationally, and that the majority of cases were symptomatic. The model also estimated that 94.8% of the Wuhan population were still susceptible on January 31, 2020. The study highlights the importance of rapid case identification and isolation to reduce the chance of onward transmission. It also shows that combining multiple data sources is valuable for analyzing the transmission dynamics of COVID-19. The results suggest that transmission in Wuhan declined in late January 2020, coinciding with the introduction of travel control measures. The study also found that the estimated reproduction number and published estimates of individual-level variation in transmission for SARS-CoV and MERS-CoV indicate that a single case introduced to a new location may not necessarily lead to an outbreak, as high individual-level variation in transmission makes new chains of transmission more fragile. The study concludes that understanding the effectiveness of control measures in different settings is crucial for understanding the dynamics of the outbreak and the likelihood that transmission can eventually be contained or effectively mitigated.