question: Studies of novel coronavirus disease (COVID-19) have reported varying estimates of epidemiological parameters such as serial intervals and reproduction numbers By compiling a unique line-list database of transmission pairs in mainland China, we demonstrated that serial intervals of COVID-19 have shortened substantially from a mean of 7 8 days to 2 6 days within a month This change is driven by enhanced non-pharmaceutical interventions, in particular case isolation We also demonstrated that using real-time estimation of serial intervals allowing for variation over time would provide more accurate estimates of reproduction numbers, than by using conventional definition of fixed serial interval distributions These findings are essential to improve the assessment of transmission dynamics, forecasting future incidence, and estimating the impact of control measures