  Early assessments of the spreading rate of COVID-19 were subject to
significant uncertainty, as expected with limited data and difficulties in case
ascertainment, but more reliable inferences can now be made. Here, we estimate
from European data that COVID-19 cases are expected to double initially every
three days, until social distancing interventions slow this growth, and that
the impact of such measures is typically only seen nine days - i.e. three
doubling times - after their implementation. We argue that such temporal
patterns are more critical than precise estimates of the basic reproduction
number for initiating interventions. This observation has particular
implications for the low- and middle-income countries currently in the early
stages of their local epidemics.
