Seeing as you’re being pedantic, I will too!
That’s definitely not how
efficacy is calculated but it’s not far off how effectiveness is calculated. Efficacy is a measure of how well an intervention works in a controlled clinical trial. You need two different interventions for your calculations, ideally drug vs placebo.
Effectiveness is a measure of “real life efficacy”. How well your intervention works after it’s rolled and used in day to day clinical practice. Usually without any comparator at all. So the Israeli data here is a measure of vaccine
effectiveness and you don’t need a placebo cohort for comparison.
What you do need is some other matched cohort, if you’re going to talk about a % effectiveness. Similar to that graph you posted yesterday. Because they don’t get placebo it’s not going to be very accurate (people will behave differently after getting what they believe is a vaccine) but it will provide some context. That’s missing here, so we’re really just going off the fact that they’re experiencing a surge, so 20 cases per 128000 seems reassuringly low. The national average over the last 7 days is 536/100k. Although regional variations will mess up any estimates of effectiveness by doing simple maths on those figures.
EDIT: I’m sure you know all this. Just chatting shit mainly for the benefit of anyone else who’s interested.