For most people, influenza is an infection that causes fever, muscle aches, headaches, severe fatigue, cough, and runny nose for several days, with some symptoms that linger for weeks. It usually offers no cause for serious long-term health concerns. For others, however, the same infection can pose much graver health risks, with consequences that may be life-threatening. The severity of influenza varies year to year.
Influenza viruses are spread from an infected person to a non-infected person by coughing and sneezing. The incubation period for influenza is one to four days, with an average of two days. Adults typically are infectious from the day before symptoms begin through five days after the symptoms appear. Children can be infectious for 10 or more days, and young children can spread the virus for up to six days before they show signs of illness. People whose immune systems are severely weakened can remain infectious for weeks or months.
Immunization against the viruses that cause influenza can prevent an infection or lessen its severity. The CDC recommends an annual flu vaccine for these groups: children 6 to 59 months; people 50 and older; people of any age with certain chronic health conditions; and people who live in nursing homes and other long-term care facilities. The vaccine is also recommended for women who are or plan to be pregnant during flu season.
The best time to get a flu shot is in early to mid-autumn. The flu shot contains killed virus, so you cannot get the flu from a flu shot. An alternative to a flu shot is an attenuated live virus vaccine that's sprayed into the nose. This vaccine is recommended for children ages 2 and older and adults up to age 49. Studies done in 2006 suggest that this version of the vaccine is more effective in children than the injectable type.
Who should get the flu vaccine
The following groups are considered high risk for developing complications from influenza. Members of these groups and people who care for them should get an annual flu shot.
Adults 50 and older
Because their immune systems are not as strong as those of younger adults, people 50 and older are more likely to develop complications from the flu. More than 80 percent of deaths from the flu and its complications—pneumonia being the most common—occur among this age group.
An annual flu shot has been shown to prevent or reduce the severity of the flu in older people. In some cases, an older person's immune response to the vaccine may not be strong enough to ward off infection completely. A flu shot, however, lessens the severity of the symptoms, often preventing hospitalization and death from complications of influenza.
Children and pregnant women
Children ages 6 to 59 months and women who will be or plan to be pregnant during influenza season should also be immunized.
People with chronic medical conditions
A person with a chronic medical condition is more vulnerable to serious complications of the flu. These are chronic medical conditions:
Chronic lung diseases, including asthma, which affects approximately 9 million U.S. children and 11 million U.S. adults
Diabetes, diseases of the heart, kidneys, or liver, or chronic anemia
A suppressed the immune system, including generalized malignant conditions, such as leukemia and lymphoma
Chemotherapy or radiation treatment for cancer
Immune suppression for organ transplants with medication to prevent rejection
Receiving high levels of corticosteroids, which are used to treat conditions such as rheumatoid arthritis, endocrine disorders, severe psoriasis, and ulcerative colitis
Those in contact with high-risk groups
Influenza can be easily transmitted via a simple sneeze or physical contact, so it's important to protect those at high risk from infection by others. The influenza virus can also be transmitted even before a person shows symptoms of infection. For these reasons, the CDC recommends annual immunizations for those who care for, live with, or have frequent contact with people in high-risk groups:
Household members, including children, and those who have frequent contact with older adults. This includes, for example, people who care for their elderly parents or grandparents.
Those who live with a person who has a chronic medical condition, such as asthma, diabetes, or heart or kidney disease.
Physicians, nurses, and other personnel in hospital, outpatient care, or nursing home settings.
Those who provide home care to people at high-risk. This includes visiting nurses and volunteer workers.
Who should not get the flu vaccine
The CDC recommends that people in the following groups not get a flu vaccine before talking with their doctor:
People who have a severe allergy—such as an anaphylactic reaction—to hens' eggs.
People who have previously developed Guillain-Barré in the six weeks after getting a flu shot.