I said in another thread that these days my brain is sufficiently addled that I can't do simple sums anymore.
So in terms of the 50% effectiveness business, in the UK:
The first shot alone is apparently (broadly) 50% effective. The second makes it (broadly) 90% effective, say Pfizer.
Originally we were going for two shots three weeks apart. That's scenario one.
Now we are going instead for two shots 12 weeks apart instead. That's scenario two.
So:
Scenario one is (let's say to make the sums "easy"), 10 million people for 3 weeks at 50% effectiveness, followed by those 10 million people for 9 weeks at 90% effectiveness. So for the first three weeks, 5 million of those 10 million may still get covid. Then for the next nine weeks, only 1 million may get covid.
Scenario two is twice as many people inoculated, but only one shot. So 20 million people at large for 12 weeks at 50% effectiveness. So for the full 12 weeks, 10 million might still get covid.
Doesn't that mean that scenario one is six million still at some risk, whereas scenario two is ten million still at some risk, over the same twelve weeks?
I think I've overthought myself into a corner here!
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