Just looked up about the flu shot and read this on the Huffington Post.
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It's tricky to predict just how long you have before immunity wanes, however. LiveScience reported that the vaccine offers protection for six to eight months, while The New York Times reported in young, healthy people "protection might last for years, even as circulating flu strains change." In fact, some medical professionals even recommend a second dose of the vaccine for people over the age of 65 or with particularly compromised immune systems.
If you want to read the whole article here's the link below.
The Best Time To Get The Flu Vaccine If You Really Don't Want To Get Sick