Mild powerlevel, but I did census work last year, and while it was positive/neutral on the whole, there were still the occasional bad homes to go to. Had the occupants threaten to call the cops on me a couple times for doing my job; even though a census worker is legally allowed to be on your property, the advice we were given was "if they threaten to call the police, just get out of there and someone will come back another day." And that was just for showing up with a short questionnaire to fill out!
Now imagine the reactions people would have to someone turning up at their door with needles ready to jab. It'd escalate much faster, guaranteed. You couldn't pay me enough to put myself at that much risk, and I'm sure that's how a lot of other prospective door-to-door jabbers would feel.