An October 8, 2018 article in FiveThirtyEight discussed how long immunity protection lasts after a flu shot and how to decide when to get one. Several studies have suggested that the effectiveness of the flu vaccine wanes over time—so the U.S. Centers for Disease Control and Prevention recommendation is not to get the vaccine too early (for example, in August) but also not to wait too long because it takes two weeks to build immunity after getting the shot.
Marc Lipsitch, professor of epidemiology at Harvard T.H. Chan School of Public Health, told FiveThirtyEight that the recent studies share methodological flaws, making it hard to accurately assess the flu vaccine’s effectiveness. He added, though, “I consider the evidence, flawed though it is, convincing enough that I think there’s almost certainly no harm in waiting.”
Read the FiveThirtyEight article: Make A Flu Shot Your Halloween Tradition