Thread by @AdamJKucharski

Given below observation, an obvious question to ask is 'did any models consider a scenario that could give insight into what a Tier 4-like change in behaviour could look like?' 1/

https://twitter.com/FraserNelson/status/1479487688973688836

If we look at scenarios that assume a prolonged return to behaviour levels seen in earlier steps of the reopening in England (e.g. Step 0, S1, S2 etc. below from 11 Dec: cmmid.github.io/topics/covid19…), models predict far lower levels of hospitalisations and deaths. 2/

Could summarise above scenarios as: 'If you want epidemic curve to look like that, this is the sort of virus characteristics and behaviour change that would be required - whether behaviour change comes from a policy, or pin hopes on spontaneous individual-level change' 3/

Remember, the virus doesn’t care whether a reduction in social contacts comes from individual-level spontaneous behaviour change or a top-down policy. So if we want to understand what's driving epidemic dynamics, makes sense to compare like-with-like as much as possible. 4/

Comparing scenarios without much behaviour change with a reality in which there was huge (and disruptive) change in social interactions creates a risk of attributing a wrong (or incomplete) explanation to the patterns we've seen... 5/

E.g. It might be tempting to attribute all difference between a baseline scenario and reality to presumed overestimation of severity (see below for some nuances on this point) rather than the lack of spontaneous behaviour change in baseline scenarios: 6/

https://twitter.com/_nickdavies/status/1473941671196905478?s=20

The distinction is important – if explanation is severity alone, implies much of the susceptibility to Omicron was never there in first place; but if behaviour played a role too, more susceptibility still remains out there... 7/

There was an analogous need to understand drivers of dynamics in summer 2020. If 1st wave had declined because of immunity (which some claimed at the time, despite available data suggesting otherwise), a terrible 2nd wave wasn't possible.... 8/

But if 1st wave decline was mostly behaviour change (which evidence suggested), it meant there was still considerable susceptibility out there, and risk of bad 2nd wave. It's not enough to just look at an epidemic curve; we need to understand *why* it has the shape it does... 9/

Over past century, epidemiology has moved from description ('I think this year's epidemic will look like last year') to explanation ('what causes epidemics to rise & decline? Why would this year be any different?) If we want to understand future of COVID, we need to do same. 10/

As you may have noticed, mobility patterns weren't exactly same as last winter – especially retail & recreation. And now seeing post-Christmas increase. Which makes it all more important to understand what shaped epidemic over past month or so, and what might happen next. 11/11