The month of July has seen Covid-19 cases in the United States increase at the fastest pace since last winter, marking the start of the latest wave of infections to afflict the nation. A new STAT analysis of Covid-19 case data reveals this new wave is already outpacing the spring and summer waves of 2020.
There are many metrics that governments, scientists, and media outlets have used to try and reckon with the Covid-19 pandemic. One of the most popular ways of visualizing Covid data has been to track the weekly average of new cases. This is pictured below.
The number represented by the line could be thought of as the velocity of cases in the U.S. It tells us how fast case counts are increasing or decreasing and does a good job of showing us the magnitude of each wave of cases.
The chart, however, fails to show the rate of acceleration of cases. This is the rate at which the number of new cases is speeding up or slowing down.
As an analogy, a car’s velocity tells you how fast the car is going. Its acceleration tells you how quickly that car is speeding up.
Using Covid-19 case data compiled by the Center for Systems Science and Engineering at Johns Hopkins University and Our World in Data, combined with data from the Centers for Disease Control and Prevention, STAT was able to calculate the rate of weekly case acceleration, pictured below.
In this chart, we see how quickly the weekly average of new cases is changing. When the values are positive, new case counts are increasing, and when the values are negative, new case counts are falling. Highlighted in red, we can see each previous wave’s intensity and duration.
Looking at the data this way is useful because the rate at which cases increase is a reasonable indicator of how intense that wave might be and how long it might last. For example, case acceleration in the U.S. reached a peak in November 2020, closer to the start of the nation’s deadly winter wave than to when cases reached their zenith in January of 2021.
This view of the data reveals that the United States is currently in the midst of a fifth wave of cases and that this new wave is growing faster than the first and second waves from spring and summer of 2020.
STAT also calculated case acceleration rates for each state and major territory in the U.S., revealing where cases are increasing the fastest.
In the last two weeks, new case counts in Louisiana accelerated the fastest in the nation at an average rate of 444 cases per week per day (2.38 cases per 100,000 people per week per day). Only 36% of the state’s residents are vaccinated, making it among the least vaccinated in the country.
By looking at the state’s case acceleration rate, we can see that cases in Louisiana are currently increasing faster than they did at the start of last winter’s wave.
Likewise, in the state of Florida, the case acceleration rate has outpaced that state’s 2020 summer wave.
In Florida, about 48% of residents are fully vaccinated against Covid-19.
Cases are increasing in nearly every region of the country, but they are not increasing at the same rate everywhere. Vaccination rates likely help explain these variations.
The five states where cases are accelerating the fastest all have vaccination rates below the national average. But consider the state of Massachusetts, where about 63% of the population is fully vaccinated.
The New York Times’ Covid Dashboard reports the state has an alarming 351% increase in cases over the last 14 days, the highest such percentage change in the nation. Looking at Massachusetts’ case acceleration paints a different picture.
While cases in Massachusetts are increasing, the rate at which case reports are accelerating is much lower than it has been for any of the state’s previous waves, and is below the national average for case acceleration.
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