Graphs like the ones below scare me. I can’t see how one can spin this as anything other than a disaster in the making. https://www.nytimes.com/interactive/2020/us/coronavirus-us-cases.html
People often complain that I’m looking at cases instead of some other metric: hospitalizations, deaths, contestants on the Bachelor who test antibody positive, whatever.
“Positive cases are determined by testing.”
“You can’t compare April to June.”
“Deaths are what matter.”
I look at cases for two primary reasons.
They are a leading indicator. Deaths lag cases by 2-3 weeks, at which point it’s too late.
Cases are the causal drivers of new cases. Cases now are the best predictor of cases in a week.
It’s true that changes in testing make it difficult to directly compare case numbers between early April and late June.
But that’s not what I’m interested in. I don’t all that much care whether we’re above the peak of the initial spike. What I care about is where we are going.
So what I want to do is compare cases in Florida today with cases in Florida a week ago. Barring extraordinary circumstances, testing doesn’t ramp up exponentially the way that cases have. On the timescale of a week, testing changes are usually a wash.
“What about deaths? Why shouldn’t we care about these at least as much as cases? Deaths are not increasing in Florida.”
Not yet, that’s true.
But deaths are a severely lagging indicator or at least three reasons.
It takes 2-3 weeks to progress from infection to death.
It often takes a week or more for deaths to be registered.
If the leading edge of a spike is driven by younger people, you need one or more rounds of transmission to reach a vulnerable population. But you will.
So the trajectory of deaths tells me about how who was infected a month ago was changing, a month ago.
I want to know how who is infected now is changing now, and what that bodes for the future.