MORE THAN one in three American states have started to reopen for business after weeks of lockdown. But has America seen the worst of the coronavirus pandemic? It is difficult to say. On the one hand, only two states—North Dakota and Kentucky—have met the Trump administration’s criteria for easing restrictions, according to health-policy experts at CovidExitStrategy.org, a new website created by former public-health officials from the Obama and Trump administrations. (The criteria include, among other things, a decline in new cases of coronavirus and other flu-like illnesses over two weeks). On the other hand, new cases of covid-19 have been falling for weeks, which suggests that the country may now be over the peak.
Identifying the peak of the pandemic using official case counts alone is difficult. First, the figures are affected by the extent of testing. As more tests are conducted, the number of new cases of covid-19 inevitably goes up. Then there is the issue of geographical variation. Though the number of new cases is falling in New York City, an early centre of the outbreak, the trend in the rest of the country is less certain. Finally there is the problem of data quality. The number of reported new cases tends to be higher at weekends and on Mondays, and lower on Thursdays and Fridays, for example. Adjusting for such seasonality can be tricky; a peak or valley on any given day can be accentuated by a number of other factors, not least variation in tests performed.
To determine whether the coronavirus has peaked in America, we built a statistical model that seeks to take account of these factors. We started by splitting the country into two groups: the states surrounding New York City (New York, New Jersey and Connecticut), and the other 47 plus Washington, DC. Then we estimated the daily number of cases in each group, using an equation that included a set of relevant variables, eg, the number of days since the first detected case of covid-19, the day of the week and the number of tests conducted on that day. This allowed us to calculate the daily growth rate in covid-19 cases while controlling for testing and seasonality.
Our results suggest that America is probably on the mend. The estimated number of new cases in the country—what government statistics might show if every person were given a test—has fallen by 40% in the New York metro area and 15% in the rest of the country over the past week. Although our model should not be mistaken for a forecast, the data available indicate that America is currently on a path to putting the first wave of coronavirus infections behind it. If lawmakers are prudent about easing lockdown restrictions, and citizens follow public-health guidelines, this could be the start of a steady recovery. But given the haste with which states are lifting the lockdown, it is far from certain that they will be spared a second wave.