Figure 3.

The effective reproduction number $R_E\left(t\right)$, sometimes referred to as $R_0$ or “R naught,” for New York fit to data with an exponential curve. $R_E\left(t\right)$ represents the average number of secondary cases because of one infected person at the beginning of the epidemic, and in order for the epidemic to decline, $R_E\left(t\right)$ should be less than 1. The points are the calculated daily effective reproduction number calculated by number of observed cases divided by the number of expected cases on a given day, while the gray curve forecasts future effective reproduction numbers that can be used in dynamic projection SIR models. Here, $R_E\left(t\right)$ went below 1 (represented by the dashed line) around 28 days.