Cases of flu tend to increase as temperatures decrease. According to the CDC, people should get their flu shots as soon as they are available.
Most 2019-2020 shots have been available since August. Most flu outbreaks happen in February and March.
The influenza vaccine takes two weeks to incubate and build antibodies. When it does kick in, the vaccine will last six months.
The young, elderly, and people with health problems should get the vaccine immediately.