That seems complicated. Something just as bad -- and simpler -- can happen by substituting a placebo (saline) for actual vaccines, which means the vaccinated are unaware they are not protected from the disease they got the shot for.
There was a
Law & Order episode based on an actual case like that:
Influency.
https://www.imdb.com/title/tt0629268/The general storyline was about a conman who created thousands of vials of flu vaccine that contained harmless saline -- nothing else. He sold the cases of vials to anyone who would buy -- distributors, clinics, private doctors, etc. The Manhattan ME noticed 16 deaths from the flu in which all the deceased had been vaccinated. The investigation from the bottom of the pyramid all the way to the one man responsible ended with him being convicted on 16 counts of manslaughter.
Even though the contents of the vaccine vials was benign, his substitution of fake vaccine for the real medicine was the primary cause of the deaths. Whether he intended to cause any deaths or not was irrelevant.
Imagine if he HAD intended to harm people. He could have put something slow-acting in the vials that wouldn't be immediately linked to the shot and which could cause sickness and death in otherwise healthy people.
The one factor that made the scam work was a shortage of actual flu vaccine. Medical professionals were happy to turn a blind eye to the origin of the fake vaccines as long as they could get it (thinking it was the real thing). The shortage is what prompted the conman to think of that scam. People were looking to buy whatever they could find from any source and at almost any price.
Now imagine an unscrupulous person (terrorist, disgruntled employee, another insider with a desire to hurt people, ...) has access to the vaccines on the assembly line and can circumvent the QA process. Tainted vials could be distributed all over the globe, and it could take a really long time before the source of the damage is identified. Meanwhile, everywhere you turn, you're being pressured to get vaxxed ...