in the the australian today, page-6

Tassie Two

Unfortunately you cant just mix them together like a witches potion, I believe from memory that the virus is programmed and/or modified for each different company....

This is why they are all using different virus's and vla has chosen to use the common cold virus which is proving to be a very good choice....as human nature says "i dont mind having the common cold put in my arm....but dont stick that herpes virus in there????..LOL

cheers