To best protect yourself from the flu, you need to get a flu vaccine every year. That’s because the vaccines change each year to keep up with rapidly adapting influenza viruses. Because flu viruses evolve quickly, last year’s vaccine may not protect you from this year’s viruses.
After you get your vaccination, your immune system produces antibodies that will protect you from the flu viruses. It takes up to two weeks to build immunity after getting a flu vaccine. After a while, antibody levels will start to decline — another good reason to get a flu shot every year.
Generally it’s a good idea to receive the flu vaccine in the fall, usually in September or October. But if you miss that timing, it’s never too late to get a flu vaccine, as it will still protect you when you are vaccinated. Also, although peak flu season is usually during winter, that can change from year to year. For example, in 2009, when the H1N1 flu pandemic hit, the largest number of cases in the United States was reported during May, June and July.
After getting the flu vaccine, a few people do develop flu-like symptoms, usually a low-grade fever that lasts about a day. In many cases, this happens because those people were previously exposed to a virus that was similar to the vaccine strain, and their immune system is already prepared to respond to it. The fever is a sign of their immune system response, not a symptom of influenza infection.
Some individuals need to be careful about receiving the flu vaccine, such as those who are severely allergic to eggs, or people who’ve had a severe reaction to a previous flu vaccine. With appropriate precautions, however, even people with special concerns can usually get the vaccine safely. If you have questions about the flu vaccine, talk to your doctor. — Daniel Maddox, M.D., Allergic Diseases, Mayo Clinic, Rochester, Minn.