Fig. 1.

(Upper) Exponential model applied to new infection and death data for Italy, Germany, France, Spain, the United Kingdom, and the United States, normalized by the total country population (54). Insets show the same data on a logarithmic scale. Both the normalized infection $i$ and death $d$ data were thresholded to comparable initial conditions for each country; fits are to the first 15 to 20 d of the epidemic after exceeding the threshold. The fitted doubling time is shown for both infections ($T_{d,i}$) and death ($T_{d,d}$) data. Data from Japan and South Korea are shown for comparison. (Lower) Dynamic reproduction number (mean and 95% CI) of COVID-19 for China, Italy, and the United States estimated from reported deaths (17) using a nonparametric branching process (18). Current estimates are as of 1 April 2020 of the reproduction number in New York (NY), California (CA), and Indiana (IN; confirmed cases used instead of mortality for Indiana). Reproduction numbers of COVID-19 vary in different studies and regions of the world (in addition to over time) but have generally been found to be between 1.5 and 6 (19) prior to social distancing.