Vaccines all work in a similar way. They produce antibodies short term, and memory T cells and other defenses long term. After antibodies wane over a period of months, Against most historical diseases like measles, etc, incubation is 10-14 days, so long term Memory T cells and other defenses kick in around 5 days and prevent infection, ultimately leading to herd immunity if most people take the shot.
Flu and Covid vaccines have same mechanisms but the diseases are different. They mutate to different variants so the short term antibodies become less effective. And they incubate in less than 5 days so long term memory cells may not kick in fast enough to prevent infection once antibodies are gone. But the memory T cells are good at fighting the infection once it sets in making severe illness much less likely.
