how long does the flu shot take effect
Reaction to the vaccine. Some people experience muscle aches and a fever for a day or two after receiving a flu shot. This may be a side effect of your body's production of protective antibodies. The 2-week window. It takes about two weeks for the flu shot to take full effect. If you're exposed to the influenza virus shortly before or during that time period, you might catch the flu. Mismatched flu viruses. In some years, the influenza viruses used for the vaccine don't match the viruses circulating during the flu season.