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After vaccination, the body’s adaptive immune response needs time to build targeted antibodies. B cells that recognize the influenza antigens must become activated, proliferate, and differentiate into antibody-secreting plasma cells. This clonal expansion and maturation process takes time, so detectable and protective antibody levels usually rise over about one to two weeks, with the peak or plateau reached within a couple of weeks. That’s why the typical timeframe is described as a few weeks.

Hearing a few hours would be far too soon, since immediate antibody production doesn’t occur unless there are pre-existing antibodies from prior exposure. A few days is still usually too soon for the full-fledged adaptive response to develop. Several months would be longer than needed for initial protection after a single vaccination. The two-week range aligns with how the immune system learns the vaccine antigens and mounts a targeted antibody response, providing influenza protection after about two weeks.