It’s one thing to have magnets sticking to arms, all it implies is the presence of magnetic susceptibility (e.g. iron). But to spontaneously create a macroscopic magnetic field requires self-organization of some kind, and more substance than could be included in a shot. My guess is that their hemoglobin is being organized. And it won’t be confined to their arm. This effect would account for blood clotting. But why isn’t it happening with everyone? Is there a prevention strategy or only a delay strategy? If it can’t be stopped, it will be fatal.
Wild speculation: If the magnetic field is being used as part of the mechanism of self-organization, applying a demagnetizer might delay the process. But by the time the effect is this large, it’s probably already gone well past the arm.
I’ve heard that some people have used strong magnets (such as https://amazon.com/gp/product/B01N8S12GM ) + time to draw something out of the injection sites.