I can't seem to find the source data on the Israel Ministry of Health website, but in Dr. Campbell's latest video, he went over an article that explains the reduced efficacy of the Pfizer vaccine against Delta variant infection (now claimed to be 39% overall) in Israel to be caused by declining efficacy as time goes on rather than any specific weakness to the Delta variant.
Take with a grain of salt until clear source data are provided, but according to the report, they looked at different cohorts depending on vaccination month and found:
Those vaccinated January now have just 16% efficacy against infection,
those vaccinated in February have 44% efficacy against infection,
those vaccinated in March have 67% efficacy against infection,
and those vaccinated in April still retain 75% efficacy against infection.
Crazy if true and we will surely start to see it play out soon in next month or so in the US as well, since our first large batch of people fully vaccinated with Pfizer completed their vaccinations in February of this year, with the vast majority completing in April, May, and June.