This graphic is an update of Figure 1 in Harris, J.E. The Subways Seeded the Massive Coronavirus Epidemic in New York City, April 24, 2020.
The figure simultaneously tracks the daily movements of two variables from March 1 through April 10, 2020. The pink-filled circles show the numbers of new coronavirus infections reported each day by the New York City Department of Health. For this variable, the vertical axis on the left is rendered on a logarithmic scale. That way, a straight-line trend would represent the exponential growth typically seen during the initial upsurge of an epidemic where everyone in the population is naïve to the infectious agent. (See Harris, J.E., The Coronavirus Epidemic Curve Is Already Flattening in New York City, NBER Working Paper 26917, April 6, 2020.)
For the same variable of newly reported cases, the horizontal axis at the bottom ticks off the date that the coronavirus test was performed. By contrast, in Figure 1 of The Coronavirus Epidemic Curve Is Already Flattening in New York City, we tracked newly reported infections in relation to the date the test results were received. The new reporting convention, which has been recently adopted by the city’s health department, has the advantage that it cuts out the delay between the date that a healthcare worker swabbed a sample from a patient’s nose and the date that the laboratory notified the department of the test result.
The second variable tracked in the figure above represents the total numbers of entries every day into any of the approximately 4,600 turnstiles located throughout New York City’s 496 subway stations. These counts are reported each week by the Metropolitan Transportation Authority (MTA). This variable is represented as sky-colored vertical bars, measured in millions of entries tallied along the vertical axis on the right side of the figure. For this variable, the horizontal axis measures the dates on which riders passed through the system’s turnstiles. While the MTA also reports turnstile exits, the data do not allow an analyst to link a particular rider’s station of entry with that rider’s station of exit.