Well that is still a bioweapon.
We don't have all the specifics so its tough to say. However this "epidemic in Ukraine seems to fit the bill. Some of the people who are sick were already vaccinated for the swine flu, perhaps the Baxter vaccination is intended to spread a virus that will ultimately kill. But it may not work as well as they want it to. The thing is no matter what, like you say, they have to maintain plausible deniability. When people drop dead immediately after vaccination that is actually a failure for them. They want a slow kill, anti-fertility is merely one way of accomplishing population reduction. If some one is sterilized through Baxter's vaccine they have essentially killed their family line.