Thread by @petermbenglish

Hi @ScienceShared and @mugevic!

https://twitter.com/ScienceShared/status/1333719276944027648

Let's say the reproduction interval is about 4 days. That's the interval between the onset of symptoms in case 1 and the onset in case 2 (on average).
1/10

https://twitter.com/petermbenglish/status/1333717670949216264

Patients are most infectious before the onset of symptoms. So, if case 1 develops symptoms, they will probably already have infected at least half of the secondary cases.

And some of them will already be infectious and have infected tertiary cases.
2/10

Few people will get tested on the day of onset of symptoms. Most of the time the onset of symptoms is identified retrospectively. After all, symptoms are mostly common to many other minor viral infections.
3/10

So it's usually at least 24-48 hours before people decide they need to get tested - by which time, most of the secondary cases have been infected, and some will already be transmitting disease to tertiary cases.
4/10

If you then await the test result (as we do in the UK) before telling contacts to self-isolate, you will clearly be shutting the stable door after the horse has bolted.
5/10

I'm not saying that TTIS cannot prevent any transmission. Just that it will not prevent most of it.

And it is enormously resource intensive.
6/10

When we dealt with the first UK citizen in Brighton, there were a huge number of contacts (and secondary cases). Contacting and advising all of them was possible because we only had a single case.
7/10

When case numbers reached double figures, we were overwhelmed, and could not do TTIS rigorously.
8/10

We have recruited many call centre workers to work for so-called-NHS test and trace; but even so, case numbers would have to be an order of magnitude lower for rigorous TTIS to be possible.
9/10

And, as I have explained, TTIS is unlikely ever to be able to prevent most of the transmission.
10/10