I see a lot of question re flu shot timing in my timeline. Since I'm on clinical service, doing one single main tweet on it here: Generally CDC recommend Sept-Oct for flu shot because you need time to build the immunity after vaccine (~2 weeks). 1/3 https://www.cdc.gov/flu/prevent/keyfacts.htm

Here is what CDC recommends:
https://www.cdc.gov/flu/season/faq-flu-season-2020-2021.htm

Antibodies after vaccination do decline over time and more so in older patients (>65yrs of age)- see duration of immunity section here: https://www.cdc.gov/flu/professionals/acip/immunogenicity.htm

You want to take the vaccine before flu activity occurs at high levels in your community.
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I would recommend taking flu shot slightly earlier than usual (sept) (particularly if you are younger) because we don't all want to rush for a vaccine at the same time during pandemic & we want to ensure we reduce flu activity right away in our communities.

If you already took your flu shot, do not distress. You will most likely be protected as CDC says:
https://www.cdc.gov/flu/professionals/acip/immunogenicity.htm