Dear Mayo Clinic: I just turned 65 and had my annual physical. This year, my doctor recommended that I receive a high-dose flu vaccine. Why do I need a high-dose vaccine?

Answer: Influenza is a viral infection that attacks your respiratory system, including your nose, throat and lungs. Influenza, commonly called the flu, is not the same as stomach flu, which is an intestinal infection.

For most people, the flu resolves on its own. But sometimes the flu and its complications can be deadly.

People at higher risk of developing flu complications include children under 5, especially those under 6 months; pregnant women and women up to two weeks’ postpartum; Native Americans; those with a body mass index of 40 or higher; people with weakened immune systems; adults 65 and older; people with chronic illnesses such as asthma, heart disease, kidney disease, liver disease and diabetes; and residents of long-term care facilities.

Although the annual flu vaccine isn’t 100% effective, it is the best defense against flu. Your age is likely the reason your doctor recommended the high-dose vaccine.

High-dose flu vaccines are like other flu vaccines in that they are made up of the flu strains most likely to cause an infection during the upcoming season, and they are given by injection. They are only approved for people 65 and older.

High-dose vaccines include four times as much flu virus antigen — the part of the vaccine that stimulates the immune system — as standard flu vaccines. This can give older people a higher immune response.

After a regular flu shot, older people produce 50% to 75% fewer antibodies than younger adults. Studies have found higher antibody levels in older adults who received high-dose flu vaccines.

One study found almost 25% fewer cases of influenza in adults 65 and older who took the high-dose vaccine.

As with any vaccination, the high-dose flu vaccine has side effects, and those who received it were more likely to develop side effects, which include headache, soreness at the injection site, muscle aches and fatigue. Typically, side effects ease after a few days.