My thoughts so far which may be severely refined as I have a lot of research ahead of me...
CARN, SVA, APT, NVAX to me.
APT is easy as that one may be undervalued anyway. (which reminds me I have to add 3M to the list as they are APT's biggest competitor I believe)
SVA you know the drill. Ditto on NVAX.
CARN has a lot of non bird flu items in the works, has the cheapest market cap of the group, and the wild card is if they get their nasal sniff vaccine it will be easier (I think?) to distribute and injest. Fast-tracking the nasal stuff, if it is approved, will be huge for the flu market in general long after avian flu is conquered.
Big pharm has the added bonus of trading options. In the unfortunate event that there is massive panic and an outbreak, these options will go up incredible amounts IMO.
Raw