Each metro area represents a different instance of the Covid-19 pandemic; we can use the varied experiences and timing of the virus in each metro area to better understand where we’re headed.
Seattle is 10 days to 2 weeks ahead of the rest of the country and signals our possible future trajectory
Growth rates are falling, and are down about half over the past two weeks. If Seattle’s experience is indicative, we can hope that the daily growth rate in the typical large metro area falls from about 16 percent today, to about 8 percent by Mid-April. This decline would increase the doubling time in the number of cases from doubling every 4.3 days to doubling every 8.6 days.
Minneapolis-St. Paul has done the best job of avoiding the pandemic; it has the lowest rate of reported cases per 100,000 of any US metro area, and is maintaining a relative low growth rate
At City Observatory, we’ve been tracking the growth of the Covid-19 pandemic across the nation’s metropolitan areas for the past two weeks. We’ve taken the data that’s usually reported at the county level (or aggregated to the state level) and produced estimates for each of the nation’s 53 largest metro areas (those with a population of one million or more). Each metro area represents a distinct instance of the pandemic, and by observing the similarities and differences between the experiences of these metropolitan areas we can gain some insights about where we might be headed. This commentary explores some possible themes.
Declining growth rates
The key to “flattening the curve” is to lower the rate of growth of new cases of Covid-19. Our attention should be greatly focused on lowering this rate of increase. Here we’ve tracked the average daily increase over the past week for the 53 largest metro areas; each line represents a metropolitan area. The good news, especially in the past ten days, is that all of these lines are sloping down: every metro area is experiencing lower daily growth over the past week than it was two or more weeks ago. The big question going forward is whether we can continue this progress, and how quickly we can drive the growth rate even lower.
Pay particular attention to the red dotted line on this chart. Here, we’ve tracked the median growth rate of these 53 metro areas (essentially, what is the growth rate that half of all metro areas are above and half below, on a given day). On March 21, the median metropolitan area had experienced about a 36 percent daily increase in cases over the past week. By April 3, two weeks later, the daily growth rate in the median metro area over the past week had declined to about 16 percent. That’s a very positive development. The key question is whether it is likely to be sustained. Digging deeper, perhaps we can find an answer.
Seattle: The leader of the pack
The first death to Covid-19 was in Washington and Seattle emerged as the nation’s , and Seattle became the nation’s first hotspot in early March. Through the first half of March, Seattle had the highest rate of reported cases in the nation. It was also one of the first places to implement stay-at-home policies. According to travel tracking firm Inrix, Seattle recorded a sharp decline in traffic levels on March 5, fully nine days before nearly every other large metropolitan area.. So in important key respects, Seattle is a week or two ahead of the rest of the nation’s cities, both in experiencing the spread of the virus and in implementing counter-measures. What can we learn from its experience walking point in the pandemic?
Here’s our same chart as above, with the values for all but one metro area faded out, and with Seattle’s daily growth values highlighted. You can see that at the start of this period, Seattle’s growth rate was about 15 percent, and that by April 3, the growth rate had declined to about 7 percent. In a sense, on March 21, Seattle was almost where the median metropolitan area is today. Seattle had about 38 cases per capita (the median metro on April 3 had 41). Seattle’s daily growth rate over the previous week was 15 percent, compared to the median metro on April 3, which had a growth rate of 16 percent).
Seattle’s experience may foreshadow the path that other cities are likely to follow. As a hypothetical, we consider what happens if other US cities, on average, tend to follow Seattle’s path in the next few weeks. If Seattle’s trajectory is a guide, we should expect the average daily growth rate in cases over the previous week to fall from about 16 percent to about 8 percent by mid-April. The slower growth rate lengthens the amount of time it takes for the number of cases to double; at 16 percent the doubling period is a little over four days; at 8 percent that extends to about eight days. If Seattle’s experience is a guide, many US cities will have between 100 and 200 reported cases 100,000 population in mid-April.
Also, because Seattle is ahead of the curve, the experience their bears careful watching. Do daily growth rates continue to to decline, or will the downward trend flatten out? If Seattle continues to see declines in its daily growth rate, that would be a very positive sign for the rest of the nation.
Who’s doing well?
One of our most useful analytical tools is this four-quadrant diagram showing the incidence of reported Covid-19 cases in each metro area (per 100,000 population) and the average daily growth rate in the number of cases over the past week. Our diagram puts the rate of incidence on the horizontal axis (metros with more cases per capita to the right), and areas with faster increases in cases in the past week on the vertical axis (metros experiencing faster growth are at the top). The horizontal and vertical lines on the chart show the average incidence and growth rate for the 53 largest metro areas.
Metros in the upper right hand corner have both a higher incidence of reported cases and are experiencing faster growth–this is the red zone. Metros in the lower left hand corner have both lower incidence of cases and slower growth than average. These cities are–at least at the time we collected the data–are suffering less from the pandemic than other cities.
The metro in the lower left-hand corner of our chart is Minneapolis-St. Paul. It arguably has managed (for whatever reason) to keep its incidence low and avoid a rapid growth in the number of cases. As we try to understand the factors that influence the spread of the virus, we might want to consider what distinguishes the Twin Cities from other American cities. (And maybe its just that reserved Minnesotans have been perfecting the art of social distancing for decades). More seriously, Minnesota also ranks extremely high on measures of social capital according to Robert Putnam, (as, for that matter, does Portland, which is also in the lower right hand corner of our chart.)
In addition to Minneapolis St. Paul, other cities that have relatively low rates of reported cases per capita and slower than average growth include Portland, San Jose, San Francisco, Sacramento, Raleigh and Austin. It’s not clear what characteristics these places have in common, but it may be worth noting that nearly all have above average rates of educational attainment and strong technology industries.
Who’s been worst hit: New Orleans and New York
Our quadrant diagram makes it clear where the virus has hit hardest. New York not only has a huge number of cases (62,000 on April 3), but even allowing for the large population of the New York metro area, it has the highest or second highest rate of reported cases per capita. New Orleans has essentially the same rate of reported cases as of April 3, roughly 570 per 100,000.
These hard hit cities, it seems, should also be the subject of close scrutiny. What is it about New York and New Orleans that caused the virus to spread as rapidly as it did. As we’ve noted, both cities had roughly the same rate of reported cases as Seattle on March 20, but since that date, have gone in very different directions. Seattle had been implementing stay-at-home policies ahead of both of these cities, which is probably a major contributor to their differing trajectories.
In an important sense, the experience of each large US metro area represents a slightly different experiment with the Covid-19 virus. It strongly behooves us to carefully study the varied timing and paths of the pandemic in each of these places to better understand how to bring the virus under control