Why Contact Tracing was Never Going to Work for COVID-19

Early in the COVID-19 pandemic, we heard a lot about "contact tracing," and an effort was mounted to raise a veritable army of "contact tracers" to contain the disease's spread (I enlisted -- took an online course via Johns Hopkins University, got certified, and theoretically could have worked as a "contact tracer" if I'd found a job opening for it -- but never actually did; Florida used student volunteers). 

Obviously "contact tracing" didn't work, and after a little while we stopped hearing about it.

Why didn't it work?

It didn't work because it can't work on an infectious disease, seemingly spread in casual contact, that's already in community spread, and which has a high R0 (R0 is the average number of other people an infected person infects).

The classic example of "contract tracing" being workable is sexually transmitted diseases like syphilis.

A patient presents with syphilis (estimated R0 of 1.5). Whoever's responsible for contact tracing has pretty much one question to ask:

"Who have you had sex with since [estimated date when the patient became infectious]?"

Most people can easily answer that question. There are exceptions (e.g. sex workers or their clients who don't know each other, people who had one-night stands and never got last names, etc.), but most people have sex with relatively few other people and know who those other people are.

Then it's a simple (although not necessarily easy) matter of contacting those sexual partners and getting them in for treatment.

Contact tracing for COVID-19 (estimated R0 of 3.32) patients would amount to "who have you been anywhere near since [estimated date when the patient became infectious]?"

Where -- everywhere-- have you been for the last X days?

Who -- everyone -- have you said hello to at the park, passed in an aisle at Walmart, sat near at a baseball game?

Most people can't remember everywhere they went over the last x days, and almost nobody can identify everyone they've been near during any such period. And even if they could, the public health establishment is still dithering over the conditions under COVID-19 is likely to be transmitted.

So, other than the obvious (close family members, people you live with or work closely with), contact tracing just really couldn't produce much useful information.  All that broadcasting "hey, everyone who was at this Walmart between the hours of 8pm and 9pm on May 10th, you need to quarantine" could do was produce needless panic.

I can't say I blame anyone for hoping early on that contact tracing could be helpful. But it was a false hope.