Sunday, December 19, 2021


It occurred to me that the talk of perhaps bringing in restrictions some time in the future was probably poorly timed, in that we are probably pretty close to the peak right now and if action is going to be worthwhile, it needs to be pretty much immediate. Having made a few comments to that end on twitter, I thought I should check out my intuition with some calculations. So here they are.

My starting point is that the Omicron variant represented 22% of tests on the 11th Dec (link) and we had about 40k positive tests on that day (link - but see additional note at bottom of post) meaning 9k tested cases which I will assume represents 18k real infections (ie 50% of infections are actually observed) and furthermore I'll assume that these infections happened on the 8th as it must take a little while to feel ill and get tested. 

I'm using a doubling time of about 2 days with an underlying R0 number of 6, and another assumption I'm making is that the population is about 50% immune. I'm ignoring the Delta infection which is small in comparison and carries on largely in parallel with Omicron.

So I initialise the model to hit 18k infections on the 8th, and ran it forwards. This is what I get with no action at all, just the natural infection profile of an uncontrolled epidemic:

32 million infections in total, with a daily peak of 2.7 million on the 27th.

If instead we were to introduce severe restrictions now, such that the underlying R0 dropped from 6 to 1.5, the epidemic would be much smaller:

A daily max of about 430k infections and only 4 million in total. Note that the underlying R0 dropping to 1.5 means the effective R value drops to about 0.75 as the population is half immune.

However the Govt seems to be slowly meandering towards the possibility of some restrictions in about a week. If we were to say Boxing Day instead, then we get:

The daily max here is 2.4 million, with the total about 16 million. So even this delayed action does cut the epidemic in half, by shutting it down rapidly from the peak. That's a bit better than my intuition had suggested to me.

The details of these calculations are sensitive to the timing of the peak of course, which depends on all the assumptions I've made. What is not in doubt is that every day makes quite a big difference to the outcome.

Edit: In the time it took me to write this post, the number of cases by specimen date on the 11th has been updated to 46k!