A lot of people have been asking how we have managed to get R below 1 (and in fact comparable to first lockdown levels?), given that we have the new variant. So I played with some numbers, and I think in London at least the answer could well be down to levels of immunity.

I am *not* saying that London has achieved herd immunity in the sense that we can relax all restrictions and expect R to stay below 1. However, people seem (understandably) nervous about discussing immunity at all, and I'm not sure that's helpful, so am going to grab the 3rd rail

I think the key thing is quite how massive the December spike was in terms of infections. I've pulled the following figures out of the (excellent) MRC-BSU model https://www.mrc-bsu.cam.ac.uk/nowcasting-and-forecasting-29th-january-2021/ - we went from 20 to 35% really fast. So, what effect might that have?

Well, if we assume that this immunity buys us a proportionate reduction in R, we can see what the R number would have been *had we stayed in a hypothetical level of constant lockdown that gave us R=0.7 on April 1st*

Note that this is purely hypothetical, and not an attempt to model R, because we didn't stay in that level of constant lockdown. And, for example, had we not opened up at all, we would have stayed closed to 0.7 fairly indefinitely and not dropped later (because no December spike)

However, of course, things are not the same as before, because the new strain arrived. I'm going to guesstimate that it increases R by a factor of 40%, and fit a plausible-ish logistic curve for how it took over (would like better numbers for both)

So, if I inflate the R number by the corresponding factor (starting at 100% of original value, growing to 140%), I get some kind of green R curve like this. Like I say, lots of guesstimated numbers but striking that they lead us to end up in a place that sort of matches case data

Of course, the delicate thing is that we can't just reopen completely straight away. I suspect that the corresponding curve under no restrictions might look something like this (3 times larger)

But the point is that going from 20 to 35% immunity might have had a big effect on R, enough to allow us to absorb the new variant. And hopefully, we might see another 15% increase like this, due to the level of vaccination so far. And then another 15%, and then another 15% ..