DEAR MAYO CLINIC: I'm 28 and healthy. I've never gotten a flu shot and have never had the flu. Do I really need a flu vaccination? My employer is recommending it for everyone, but I'm hesitant. I've heard some people get sick from the actual vaccination.

ANSWER: The U.S. Centers for Disease Control and Prevention now recommends that everyone 6 months of age or older be vaccinated every year against influenza. Being young and healthy does not protect you against getting the flu. Even someone like you, who has not had influenza in the past, should still get an annual flu vaccine. In some cases, people may develop minor flu-like symptoms after getting the vaccine. But the flu vaccine cannot give you the flu.

Influenza is a viral respiratory infection that tends to come on suddenly. The influenza virus is a systemic virus. That means it circulates throughout the body in the bloodstream. Symptoms typically include fever, aching muscles, chills, sweats, headache, feeling tired and weak, coughing and nasal congestion.

Influenza can cause complications, such as sinus and ear infections, bronchitis and pneumonia. These complications, particularly pneumonia, can be especially dangerous in young children, pregnant women, older adults and people who have chronic medical conditions, such as asthma, epilepsy, kidney disease or liver disease, among others. Getting an annual flu vaccine is the most effective way to prevent influenza and its complications.

It is important to note, however, that even though young children and older adults are most vulnerable to complications from the flu, the severest forms of flu that we have seen have not affected those groups the most. Instead, the most infectious and serious strains of influenza -- such as the strain that caused the 1918 worldwide pandemic -- have more often affected young, healthy adults. Most cases of death associated with severe flu strains have been in younger adults who were otherwise healthy.

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.

(Medical Edge from Mayo Clinic is an educational resource and doesn't replace regular medical care. E-mail a question to medicaledge@mayo.edu. For more information, visit http://www.mayoclinic.org.)