Of particular interest is the question of whether transmission is supercritical, meaning that R > 1, in which case the epidemic can increase in size, or is subcritical, meaning that R < 1, in which case it will fade out. A time-varying reproduction number, commonly written R, is the average number of cases infected by a given case over the course of that individual’s disease progression. We estimate transmission rates of COVID–19 using reproduction numbers. This is a daily updated tracker website for COVID–19 case counts and transmission rates in California and in the United States.
