What's your best guess?
Some combination.
Prior to this oubreak, they’ve been safe using enhanced bsl3 with a bsl4 virus. They are VERY careful and meticulous.
If a particular suit has a defect rate at 1 per 1000 and you only use 750 of those in a particular outbreak you may not ever ‘see’ the exposure and subsequent illness from that defect rate.
For example.
There’s also the issue of a vastly increased number of volunteers and patients. They’ve previously only treated patients in hospitals with a max of 20 patients. That was the largest number they felt was safe for their people. IIRC that has been increased to 100. There was a ‘reason’ for the 20 patient limit.
Plus there are undoubtely ‘new’ people doing the training and infection control who may not have the experience overall that was present in those positions in previous outbreaks just due to the much larger size of this outbreak.
I’ll defer to the biologists who say this strain may be different in subtle ways. Or maybe not.
etc.
If their suits are being manufactured in China, that may be part of the problem.