According to my reckoning the sequence is as follows: BIOTA ->...

According to my reckoning the sequence is as follows:
BIOTA -> Biota Pharmaceuticals 5.567:1
Biota Pharmaceuticals -> Aviragen Therapeutics 1:1
Aviragen Therapeutics -> Vaxart 11:1

So if you start with 100,000 shares in Biota (before moving to the US) end up with
Biota Pharmaceuticals 17962 shares
Aviragen Therapeutics 17962 shares
Vaxart 1632 shares

Unlike some smarter (or perhaps more decisive) people who sold before the move the US, I still have the remnant Vaxart shares.
Although AST stopped forwarding me any documentation after the Vaxart merger.

As noted, Vaxart has been plugging coronavirus, which I think can be safely ignored, and before that the influenza vaccine, which caught my eye, and rekindled an interest to find out what what happened to my Vaxart shares. The last communication from AST was a cheque for $2.43, but it was enough for AST to accept that I have a claim. They have given me an "account number" which should enable me do something other than wonder what happened.