My data comes from Bing's Covid-19 tracker. I focus on active cases (active = confirmed - recovered - deaths). Neither the dead nor the recovered can spread the virus - that's why active cases are the relevant factor. My earlier blog post was about total confirmed cases, but when I turn my attention to active cases, I still get the same result: the curve started to flatten in early April.
Here is the graph of active cases in the US.
You can see the exponential growth. As I explained earlier, when you have exponential growth, it's better to look at the log graph:
It's still spreading, but not as rapidly as before.
When examining the data, I asked, "When did the curve's slope change?" I found that it changed several times.
Time1 is before March 20. Time2 is from March 20 - April 7. Though this was during the lockdown, the disease was actually spreading more rapidly than before. However, it can take up to 2 weeks before symptoms appear, so this increase reflected cases that had started earlier. Time3 is April 8 - April 29. Here we see the curve flattening: the coefficients (circled in red) started shrinking. However, they were still positive. Overall, that means that the virus was spreading but the growth rate was slowing down. Time4 is April 30 - May 9. Some states had started reopening by then. Nevertheless, the growth rate kept slowing down. Right now, we are in Time5: May 10 - present. The growth rate is even slower.
This analysis does *not* prove that the reopenings were a success or that shutdowns were unnecessary. A longer shutdown would have driven down the growth rates more quickly, but we don't know if that benefit outweighs the economic costs. That question is too complicated for a single blog post. I have one piece of the puzzle: the curve is still flattening in spite of the reopenings. Other researchers will have to fill in the rest.
When I looked at the data more closely, I found that many regions are not reporting the number of recoveries. So the true number of recoveries is higher than reported and therefore the true number of confirmed active cases is lower than reported. This does not overturn the main conclusion. The curve is flattening even more than the graph shows.
ReplyDeleteAnother factor to consider: many people are asymptomatic so the number of confirmed cases is an underestimate. However, testing has increased recently, so the number of confirmed cases should be converging to the true number of cases. Thus, if the number of confirmed cases is rising, that doesn't necessarily mean that more people are getting sick - it could be that we are just getting better at detecting it. Increased testing can make the curve appear to be getting steeper even if it isn't. But the curve is flattening in spite of this. However, it is possible that the true number of active cases is actually falling even though it appears to be rising.