It takes about two weeks post-vaccination for your immune system to build flu-fighting antibodies, McBride says. Once this happens, your risk of catching the virus drops by up to 60 percent.
The flu can sideline you for about as long, so athletes are wise to take their training into account. If you're prepping for a fall or winter competition, get the shot as soon as possible to ensure your body is primed for health throughout your training plan, McBride says.
The bottom line:
Get vaccinated after you’ve done that day’s workout and follow it with a rest day, McBride says. Some people experience general malaise and soreness at the injection site that lingers for 24 to 48 hours. Never schedule it within a week of a race.