About 2 weeks
After a person receives a flu
vaccine, his body starts to identify the virus
(if live vaccine) or pieces of virus (if non-live vaccine) as foreign.
Then his immune system, with its white blood cells, lymph nodes, and spleen, remembers the virus's parts and makes antibodies against them. In about 2 weeks, his body should be able to recognize and destroy the 3 flu virus types in the annual vaccine.