Nobody knows for a fact what happened after the futility on PFS was known. There are some facts that provide clues though.

 

First, NWBO in 2017 said clearly they were waiting on 2 more OS events to start the analysis. That would be consistent with the then defined planned final OS analysis.

 

Then, with no explanation, they changed that to they were waiting for "sufficient" OS events to perform the analysis.

 

Then, prior to lock, they went to external controls.

 

OK, the obvious answer is that they performed the final OS analysis as planed in late 2017. It failed (we know that HR would be over 1.0). Note that NWBO has never denied any unblinded analysis were performed.

 

At this point they likely held out hope for a long tail separation. So they waited on that. And when that proved hopeless by the end of 2018 they gave up and went to the ECA farce.