The influenza vaccine for each season usually becomes available in the late summer well before the flu season officially starts. Once the vaccine is given, the immune system reacts by making antibodies. It is crucial to know that the vaccine does not prevent someone from getting the flu. You can still be exposed to the virus, and it can still enter your body. However, if someone is exposed to the flu later in that season, the heightened immune system can recognize the virus early and stop it from progressing into a more severe illness. This immune process following vaccination usually takes a couple of weeks, so it is best to get the flu shot early in the season.