Why does the Pfizer vaccine seem to work significantly less well In Israel

 Data (1) are available regarding effectiveness of   the Pfizer 2 dose mRNA vaccine in Israel,Canada and England/Scotland.

As regards the vaccine effectiveness  (VE) against the Delta variant  there is a wide difference  in the  values for VE for symptomatic disease .For Canada and England/Scotland there are  values of 87 % and 89% versus 64% for Israel.

Both Canada and England/Scotland administered the vaccine not according to the time frame used in the clinical trails for  Pfizer. Due as in part to a  shortage of vaccine in Canada a decision was made to give the second short 4 months after the first .

 In England/Scotland  a similar decision was made to postpone the second shot to maximize the number of  people who received the first shot as the alpha variant was surging at that time.This was done with the realization that they using a dosing schedule different from that used in the clinical trials which is inconsistent with standard method of applying clinical trial results to clinical practice. The British decision was controversial. Some suggested that a partially immune population ( i.e many folks with only a single jab) along with a surge in cases was a set up for a  perfect storm of super surging.

  In Israel, which was not facing a vaccine shortage,the decision was made to give the vaccine according to the time schedule that was carried out in the clinical trials.Also in the US The mRNA vaccines were given according to the time schedule used in the trials.So, will the data for the US show a pattern more like Israel or\more like Canada?

One explanation for the international disparity in vaccine effectiveness  is that the longer delay between doses of the mRNA vaccine results in a more robust  protective response. Could this be another example of "sometimes it is better to be lucky than good"? Perhaps confounding the comparison is the fact that the degree to which Delta was prevalent in the populations varied, i.e.  possibly more Delta at the time of the Israel data compilation. 

1) I based this blog  in part on a slide presented by CDC on or about July 30, 2021 which I believe in turn was derived from several sources including Sheikh,  a. SARs-COV 2-Delta in Scotland.Lancet vol 397 p 2461,June 26 2021 

and an article by B Lopez et al Effectiveness of Covid 19 vaccines against the B.1.617.2 (Delta) variant NEJM July 21, 2021