SARS-CoV-2 rapidly spread from a regional outbreak to a global pandemic in just a few months . Global research efforts have focused on developing effective vaccines against COVID-19 . However, some of the basic epidemiological parameters, such as the exponential epidemic growth rate and the basic reproductive number, R 0, across geographic areas are still not well quantified . Here, we developed and fit a mathematical model to case and death count data collected from the United States and eight European countries during the early epidemic period before broad control measures were implemented . Results show that the early epidemic grew exponentially at rates between 0.18-0.29/day (epidemic doubling times between 2.4-3.9 days). We found that for such rapid epidemic growth, high levels of intervention efforts are necessary, no matter the goal is mitigation or containment . We discuss the current estimates of the mean serial interval, and argue that existing evidence suggests that the interval is between 6-8 days in the absence of active isolation efforts . Using parameters consistent with this range, we estimated the median R 0 value to be 5.8 (confidence interval : 4.7-7.3) in the United States and between 3.6 and 6.1 in the eight European countries . We further analyze how vaccination schedules depend on R 0, the duration of protective immunity to SARS-CoV-2, and show that individual-level heterogeneity in vaccine induced immunity can significantly affect vaccination schedules.