As we track the scale of outbreaks in countries around the world and look for insights into what will happen in the United States, one key metric has been the number of days between a full lockdown and when new infections and deaths peaked. For that I’ve been trying to make sense of just when New York City – the center of the outbreak in the US – locked down. There’s no simple answer since the city slowly hunkered down in phases. During the second week of March the city government began encouraging businesses to start work from home for employees who were able to do so. On Sunday, March 15th, Mayor Bill DiBlasio announced that the public schools would close the following day. On March 20th, Governor Andrew Cuomo ordered a statewide “pause” , which is New York’s version of what in other states is being called a “shelter in place”.
But how much impact did these different suggestions and orders have on reducing mobility, density and spread in the city?
One way to get at this question is the city’s subway system, in many ways the lifeblood of the city. It is not only a clear vector for the spread of COVID-19. It is also one of the best measures of activity in the city in general.
I was able to get daily subway ridership data for March from a source in city government. I took those numbers and used the average weekday ridership in the subway system (5,437,587) as a benchmark to compare them against. (I used average weekday numbers from 2018, which are the most recent available on the MTA website.) These tell us when and how quickly ridership declined as the COVID-19 crisis overtook the city.
Here’s what they show.
Using the 2018, benchmark, ridership was actually ever so slightly above average for three days out of the first week of March. 101% on Tuesday, Wednesday and Thursday. During what was likely the critical week of spread, ridership remained remarkably high. The following Monday (March 9th) ridership was at 91% and only fell to 64% by Friday (March 13th). It was only on the following Friday (March 20th) when ridership fell under a quarter of normal ridership (24%). By Monday, March 23rd, it stood at 13%.
Eventually, statisticians will use data like this to chart the outlines of the spread through New York City. But just eyeballing the numbers, it seems clear that activity in the city remained pretty much business as usual for the first half of the month – down only to about 2/3rds ridership by the second Friday of the month (March 13th). And there’s nothing you could call a dramatic decrease in activity, mobility and density until the Friday after that (March 20th).
To the extent we are trying to map the length of time other countries have had to wait between going on lockdown and a peak in new infections we likely have to date that no longer than five days ago and perhaps even more recently than that. Even into the third week of the month, as the city braced for a catastrophic surge of COVID-19 cases in the city’s hospitals, ridership remained comparatively high – more than enough density to ensure dramatic spread in a city in which the epidemic was already hitting hard.