Monthly Archives: May 2020

Shrill, but true.

CDC graph improved by Down With Tyranny:

All too credible with so many states deciding to give up on anti-pandemic rules while COVID-19 is still just as widespread

To be fair, no one knows what the true R0 (the rate of reproduction of the infection) is for the US; indeed in a country this size one might more reasonably speak of different R0s for different parts of the country. But even if R0 is currently just below one, which it might be given the US infection rate has plateaued and even trended down a tiny little bit, that’s thanks to all the state-imposed ‘social distancing’. Change the rules and the infection rate surely will go back up again.

I live in a state in which all but the very southern part is, our governor says, about to “reopen”. Looking at that graph at the top, it seems all too likely that we’re not learning anything from history.

No, this isn’t a cheery post. I want to discuss three aspects of the news that three Trump-Pence staff members have tested positive for COVID-19. (That’s Trump’s military valet, Pence’s press secretary, and now a “personal assistant” to Ivanka Trump.)

The first point is that these results strongly suggest the folly of having staff go around without masks for fear someone might take their picture.  It’s really dumb.

The second point is that these results show the complete hypocrisy of the Trump administration’s failure to go all-out for a national max testing policy: the White House will respond to this news by doing more testing — something it says the rest of the country doesn’t need.

As the links above show, both these points are getting some airing.

But there’s a third point I haven’t seen in the news yet: no test is totally reliable. I read that some people in the White House are being tested every day.  Let’s assume that the White House has the best test.  What’s the rate of false positives for asymptomatic people? I can’t figure it out. I read that, at least under lab conditions,  Abbot’s new test for people who have had symptoms for a couple of weeks is very very accurate: 99.9% specificity, or about one false positive out of a thousand healthy patients, and 100% sensitivity, or a complete lack of false negative results in patients confirmed to have had COVID-19.  But that’s for people with full-blown disease, and also it’s not clear if anyone is using it yet.

What the false positive rate might be for asymptomatic people will vary with the test, and the quality of the implementation.  If it’s 99.9% then ignore all of what follows. But suppose the accuracy rate is ‘only’ 99%.  In other words, suppose that 1 out 100 flagged as positive are in fact not carrying the virus.  What are the odds of a false result if someone is tested every day?

The way you work that out, if I remember Freshman math, is to take the odds of the thing not happening (.99), and multiply it by itself for the number of events.  So (.99) to the 30th power gives you the odds that all that month’s tests will be accurate, which google tells me is about .74.  So there’s about a 1 in 4  chance of an erroneous result if we use a test that is 99% accurate on one person for 30 days.  That’s pretty high. Increase the number of people being tested daily, and the odds of a false positive on someone go up quickly.

So maybe they don’t all have it.  But it’s still very likely that at least some of them do, and given the no-mask rule, there’s a quite decent chance they will have exposed someone else.

That said, in the grand scheme of things, a 1% false positive rate is not much to worry about — the victim quarantines unnecessarily, but no one else is harmed. It’s the false negatives that are the worrying problem, because they allow the unknowing to go out and spread the disease.  And we also don’t know what the false negative rate for asymptomatic persons is for whatever test the White House is using.  Want to bet it’s not below 1%?