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1/ A sense of why, even if cases aren't rising, more restrictions may be needed outside tier 4. Here is a simple model of a region that starts off in week 1 with 95% old variants, and 5% of the new variant.
2/ Here R for the old variants is 0.9, due to the tier 2 measures. This means that for the first 3 weeks cases decline. Things look fine. But under the surface the make up of those cases are changing. The new variant (in this case modelled with 70% greater R) is taking over.
3/ By week 8 it represents the majority of the population and so there is strong exponential growth. And at that point driving R below 1 is really challenging, so cases may be locked in at a high rate.
4/ If we reduce the initial R to 0.8, by increasing restrictions, the take-over still happens (to me that seems unavoidable). But in the meantime, there has been much less increase, and so we enter exponential growth at a much lower starting point.
5/ And if we increase measures then, we may be able to lock in cases at a lower rate.
6/ By the way it's unlikely that many regions have an R as low as even 0.9 at the moment (it's a lower bound for some) https://www.gov.uk/guidance/the-r-number-in-the-uk
