Patrick Vallance, the government's chief scientific adviser, says that the policy is to let 60% of the country get it. Here is my (no doubt flawed in many respects) understanding of that policy. If on average each person infects 2.5 others in a population with no immunity, 1/
then in a population with 60% immunity, and therefore 40% susceptible, that ought to drop down to 1, the ratio at which it no longer spreads exponentially.
It sounds heartless to let 60% get the disease, but the alternative is to stop its spread at some earlier stage, 2/
It sounds heartless to let 60% get the disease, but the alternative is to stop its spread at some earlier stage, 2/
and risk some small outbreak leading to a whole new explosion of cases at some point in the future. So basically they are calculating that it's almost impossible to avoid 60% of the population (or thereabouts) getting it, at which point the focus switches to ... 3/
making that happen in the least damaging way possible. Here a major question is when to adopt strong social distancing measures. I define these as measures that reduce the ratio to below 1. There is some value in weaker measures too, as I have already explained, 4/
but I'll focus on the really strong ones, which cause the number of new cases (as opposed to number of confirmed new cases -- there is a significant lag between the two) to start falling.
I'll follow @ColinTheMathmo's rough estimate that the number of cases -- 5/
I'll follow @ColinTheMathmo's rough estimate that the number of cases -- 5/
multiplies by 100 every 20 days. I'll also assume it takes about 10 days to get from being infected to needing significant hospital treatment if you need it. (These numbers can be changed of course.) Then if we've done nothing up to now, we can expect 6/
the number of people needing significant hospital treatment to multiply by 10 in 10 days' time, and if we introduce lockdown measures now, we can (broadly speaking) expect the number of new cases to decline rapidly in 10 days' time. 6/
Those numbers are all a bit dodgy, but they point to the conclusion that the right time to introduce strong measures is when hospitals at are some quite small percentage of their full capacity with respect to beds, ICU beds, ventilators, etc. 7/
If we move too soon, then we risk a second wave. If we leave it too late, then hospitals will be overwhelmed. There will of course be mathematicians modelling this properly, and it sounds as though the government is listening to them. 8/
I find it tempting to agree with people saying we must act right now, but they aren't definitely right: getting the timing right is a delicate balancing act. I hope policy makers will err on the side of caution, but not excessively so. 9/
And I trust Patrick Vallance more than e.g. Nigel Farage or Rory Stewart on this. 10/10
I've since thought some more, and I see more clearly the costs of the herd-immunity strategy, so I've written a thread with more rough estimates, this time suggesting that herd immunity can't be achieved without overwhelming the hospital system.
