Look at the insert. You can see how cases in the active (red) and placebo (blue) change over the three weeks post dose one.
Efficacy = (difference between blue and red lines)/(height of blue line)
The average efficacy isn't so far from 30% at 21 days when dose 2 happens. But you can see how the lines have separated, and forward efficacy could be expected to be much better.
And this is for the average population in the Pfizer trial; the population vaccinated initially In Israel will be much more vulnerable and potentially have a weaker response to vaccine.