- Influenza, commonly called “the flu,” is caused by the influenza virus, which infects the respiratory tract.
- The virus generally spreads from person-to-person when an infected person coughs or sneezes.
- Compared with other respiratory infections like the common cold, the flu can cause severe illness and lead to serious, and life-threatening complications in all age groups.
- Typical flu symptoms include fever, dry cough, sore throat, runny or stuffy nose, headache, muscle aches, and extreme fatigue. Children may experience gastrointestinal problems like nausea, vomiting, and diarrhea but such symptoms are not common in adults. Although the term “stomach flu” is sometimes used to describe gastrointestinal illnesses, this is caused by other organisms and is not related to “true” flu.
- There are several common misconceptions about the flu including:
- Flu is merely a nuisance.
Flu is a major cause of illness and death in the U.S. and leads on average to approximately 20,000 deaths and >110,000 hospitalizations each year.
- Flu vaccine causes the flu.
The licensed flu vaccine used in the United States, which is made from inactivated or killed influenza viruses, cannot cause influenza infection and does not cause influenza illness.
- Flu vaccine is not very effective.
When the vaccine viruses and circulating viruses are well matched, vaccine can be very effective. However, flu vaccine only provides protection against influenza. People who have received flu vaccine may subsequently develop a respiratory illness that is due to another virus, but is mistaken for flu. In addition, protection from the vaccine is not 100%. Studies of healthy young adults have shown flu vaccine to be 70% to 90% effective in preventing illness. In the elderly and those with certain chronic medical conditions, the vaccine is often less effective in preventing illness. However, the vaccine is effective in reducing flu-related hospitalizations and deaths among older adults.
By far, the most common side effect of flu vaccine is arm soreness and swelling at the site of injection. Some people, usually children who have not been exposed to influenza virus in the past, may have fever and body aches after vaccination. These symptoms, if they occur, usually start 6-12 hours after vaccination and can continue for 1 or 2 days.
Less common side effects that can occur after vaccination include allergic reactions (particularly in people who have a severe allergy to eggs), and Guillain-Barr? syndrome (GBS), a severe paralytic illness. In 1976, swine flu vaccine was associated with an increased number of cases of GBS. Influenza vaccines since then have not been clearly linked to GBS. However, if there is a risk of GBS from current influenza vaccines, it is estimated at 1 or 2 cases per million persons vaccinated.
- Flu is merely a nuisance.
- Influenza viruses continually change over time, and each year the vaccine is updated to include the viruses that are most likely to circulate in the upcoming influenza season. The influenza vaccine (flu shot) that has been produced for the 2000-01 flu season contains three influenza virus strains designated A/Panama, A/New Caledonia, and B/Yamanashi.
- In addition to flu viruses, other respiratory organisms also frequently circulate during the same time period and can cause similar respiratory illness. For example, respiratory syncytial virus is the most common cause of severe respiratory illness in young children.
- The best time to get a flu shot is from October through mid-November. However, shots can be taken at any time during flu season. It takes 1–2 weeks, after receiving the shot, for a person to develop protective antibody.
- Flu vaccines are 70%–90% effective in preventing influenza among healthy adults. Among elderly or people with chronic conditions, the vaccine may be less effective in preventing disease than in preventing serious complications and death.
- The most important major group who should receive flu vaccine are people 50 years or older; residents of nursing homes; children and teens on long-term aspirin therapy; women who will be in their second or third trimester during flu season; and people of any age with chronic diseases of the heart, lung, and kidneys, or who have diabetes, immunosuppression, or severe forms of anemia. The second major group who should be vaccinated against flu are those who are in close or frequent contact with anyone in the high-risk groups listed above. These people include healthcare personnel and volunteers, and people who live in a household with a high-risk person.