Just an idle thought..
Ask them what the side effects are of the 'vaccine' they are taking. If they say 'none' then simply ask them how many pharmaceutical products have ever had zero side-effects.
I think this might have a chance of disrupting the signal for common sense to kick in, maybe. Worth a shot?
what is used as a placebo for a vaccine test?
Likely another vaccine.
Another vaccine. An existing meningitis vaccine was used as the "placebo" for the covid vaccines in at least one study.
https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(20)31604-4/fulltext
Saline solution would be cheapest.