I can't answer this fully, but it seems that it has already been answered. One reason is a severe allergic reaction. OH had this after the Pfizer in December '20 and it redoubled when he had the 2nd dose in early Jan. 2021. His G.P. has said that he must not have a booster.
The reaction is still with him, but much weakened. It took the form of a whole body eruption of an itching rash which stopped him sleeping and took months to control. He still has the rash visible under his skin in certain parts of his body (notably his chest) but thank goodness it no longer itches.