Testing Targets and Intensifies Social Distancing on the Infectious

I’ve been pounding on the need for fast, frequent testing but it’s clear from some of the comments to The Beginning of the End that I have failed to convey some fundamental points. A seemingly sophisticated objection is to note that given background prevalence rates even a fairly specific test will result in a high fraction of false positives among those who test positive. (This is the standard Bayesian doctor puzzle.) It’s nice to see people doing the Bayes calculation but some of them are then drawing the wrong conclusion. Let’s spell it out.

Suppose that the numbers are such that 50% of the people who test positive actually are negative. That sounds bad and it’s not great for a diagnostic test but it’s good enough to be a massive help in a pandemic. To see why, just imagine that you could easily identify people who had a 50% chance of being infectious. That’s a very useful piece of information! If just this group were to intensify social distancing for a week or two the pandemic would end quickly.

In essence, testing allows us to target and intensify social distancing on the people who are most likely to be infectious. Suppose that 10 in 1000 people are infectious (a 1% infectious rate) and that all 1000 are doing some social distancing to protect ourselves from the 1%. If we test and 20 people test positive (10 infectious and 10 not) then 980 people can return to their lives and only 20 need to intensify social distancing. The pandemic ends quickly.

We could cut the number quarantining even further by retesting using PCR and that’s good but not necessary. Also note that the 20 who test positive were already social distancing, albeit perhaps less carefully than ideal, so the additional cost is low and intensifying social distancing on the infectious reduces the transmission rate.  I have ignored false negatives to focus on a key issue. False negatives will mean some transmission still occurs but that will be picked up by more frequent testing.

The takeaway is that when you are blind, you don’t need 20/20 vision to be much better off.


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