NEW YORK (NYTIMES) - If the rate of growth in coronavirus cases in the New York metro area continues, it will suffer a more severe outbreak than those experienced in Wuhan, China, or the Lombardy region of Italy.
There is no guarantee, of course, that current trends will continue. What has happened to this point can't be used to predict what will happen next. It is possible that social distancing will soon slow or arrest the growth of cases.
But what can be said is that the New York metro area has had less success in flattening the curve, at this point in its outbreak, than Wuhan or the Lombardy region did at the same point in theirs. And some other American metropolitan areas appear to be on a similar path.
In the early stages of an outbreak, the population size doesn't matter - one infected person will probably infect a few people, whether that person lives in a metropolitan area of 100,000 or one of 10 million. But as an epidemic progresses, the number of cases per capita can provide a good measure of the prevalence of coronavirus in a community. Per capita measurements also give a sense of how strained a community's health care system has become, since larger places tend to have more medical resources.
To make useful per capita comparisons, we've focused on metropolitan areas instead of countries or US states. That's because metropolitan areas roughly correspond with the regions where the virus might spread quickly among families, co-workers or commuters. Health resources also tend to be local.
Our tables include numbers from Lombardy and Wuhan to provide a benchmark for metro areas in the United States. The comparisons are illustrative, but not exact. Those outbreaks have been going on longer, which means their case numbers are spread over more time. In most of the US, cases are from only the last month.
The number of confirmed cases is an imperfect measure of what we really care about: the prevalence of the virus in the population, and therefore - if it is early in the epidemic - how many people are sick or may be contagious. The limited availability of testing in some places means that many people with coronavirus won't be counted among the confirmed cases. And the varying rates of testing across states and countries make it hard to compare the number of confirmed cases in different regions.
Examining deaths can allow for a more direct comparison between communities, since it avoids many of the problems with variable testing. Testing differences matter less in measuring deaths because in most places with established outbreaks in the United States, the sickest patients are getting tested. That may be less true in other parts of the world: Patients who die outside hospitals in Britain and Italy have, in some cases, been omitted from official data.
But measuring only deaths has drawbacks, too. We know that the death rate from coronavirus differs depending on the age and health of the populations affected and the availability of medical resources, like ventilators. That means that per capita rates may look high in places where the virus has infiltrated nursing homes, for example, even if it has not spread widely through the rest of the community.
Because patients who die of Covid-19 tend to be sick for weeks first, counting deaths may also understate the current size of the outbreak in a given place if it is growing quickly.
To assess the possible future of the outbreak, it's helpful to look not just at the number of cases but also at how quickly they are increasing.
Public health officials have been talking about the value of social distancing measures as a way to "flatten the curve" of the epidemic. Such a flattening would mean that the rates are falling, eventually to zero. New York's current growth rate is just over 30 per cent, suggesting that its curve remains quite steep, and that the disease is continuing to spread rapidly throughout the region.
In some other places, like Baton Rouge, the growth rate is high, but the number of cases is still low. That means the community may still have time to flatten its curve before the outbreak becomes widespread. But communities with a lot of cases and a high growth rate are on track to have a serious problem. A high growth rate on top of a large number of cases means that a still larger number of people are on track to become ill or die.