Special snowflakes who never get sick and never get vaccines aside...
The flu vaccines are year-specific. Last year the CDC recommended holding off a little while to get it, because they were expecting the season to start late. The vaccines don't just have a shelf life (how long they can stay in the vial before it has to be thrown away) but they also have a half-life (how long it takes to wear off in your body until it's only 50% as effective as it was when it was injected).
Yes it can last up to 9 months. But it starts losing its effectiveness several months sooner than that. If the flu season is expected to begin in November, and last as late as March, and you get the shot this week - in mid August..
It means it'll be 2 months in when flu season starts. By the end of flu season you'll have been vaccinated for 7 months and your vaccine will be less than 50% as potent as it was when you got it.
If you wait til September or early October, you stand a much better chance of enduring the flu for the entire season.
ON THE OTHER HAND - if the CDC says this year's going to be an early season - then get it ASAP.
It really depends on the year.
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