The Chinese are on the record as saying North America would need to be depopulated in order to conquer it, and they would need to figure out a way to do so with biologicals without inciting us to nuke the shit out of them. In the same speech (Chi Hoisin ~2005) it was commented that they could suffer the loss of hundreds of millions as long as the party survived.
Having something with a 5-20% mortality rate get loose starting there is a pretty decent plausible deniability scenario...
Well the issue is that many of the biologicals would be latent at stages and bloom forth again in various mutations as conditions arose. The Chinese would be damaging themselves....vaccinations or not. And we would probably nuke them anyway even with a reduced population knowing that China hit us with biologicals.