How long should a bruise or knot at an immunization injection site last for?

Several days perhaps: A local bruise or hematoma and swelling are not uncommon near injection sites. A small vessel (vein) was likely traversed by the needle upon tissue penetration. The best indicator of resolution is decreasing pain, swelling and fading of dark discoloration from black, purple, and blue, to brown and yellow.

Varies: Usually a few days.Rarely for several weeks.