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Influenza

Fact Sheet

What is Influenza?

Influenza ("the flu"), a highly contagious viral infection of the nose, throat and lungs, is one of the most severe illnesses of the winter season. In the United States, the number of influenza cases usually is highest from December through March, but the peak of influenza activity has arrived as early as October and as late as May in some years.

Influenza spreads easily from person to person, primarily through coughing or sneezing. A person can be infected and spreading influenza for several days before his or her own symptoms start, so it is not always possible to avoid getting influenza just by avoiding people with symptoms. Unlike most other common respiratory and stomach infections that are often called "the flu," influenza can cause more severe illness and can result in complications leading to hospitalization and death, especially among the elderly. Between 5 and 20 percent or more of the U.S. population (15 to 60+ million people) get influenza every year; 200,000 are hospitalized and about 36,000 die from influenza-related complications.

Prevention

Getting influenza vaccine every year is the best way to prevent influenza. While the best time to be vaccinated is October or November, vaccination continues to be useful in December, January and even later because the virus continues to circulate and cases may not peak until February or March (or even later some years). Because influenza viruses change from year to year, it is important to get vaccinated against influenza every year.

While vaccination is the primary means of preventing influenza infection, antiviral medications may be used effectively in certain situations, such as during institutional outbreaks. Four licensed influenza antiviral drugs are available in the United States: amantadine, rimantadine, zanamivir and oseltamivir.

Zanamivir and oseltamivir were approved in 1999 for treating uncomplicated influenza infections and so far, there has been no evidence of resistance of the influenza virus to these drugs. They need to be given within two days of illness onset to be effective. Amantadine and rimantadine, are the other two antiviral drugs. When taken within 48 hours of the start of symptoms, these drugs can reduce the length of uncomplicated influenza A illness by approximately one day. One concern with these drugs, however, is that influenza viruses develop resistance to them.

Symptoms

Typical symptoms of influenza are high fever (101*F - 102*F) that starts suddenly, chills, cough, headache, runny nose, sore throat, muscle aches and joint pain. Other symptoms include moderate or severe discomfort/depression, extreme tiredness and sensitivity to light. Children may have symptoms that are uncommon in adults, such as diarrhea, vomiting or nausea.

If there are no complications, the fever generally lasts two to three days and seldom more than five. However, aches, depression and lack of strength can linger for several weeks.

Who Should Get Influenza Vaccine?

The U.S. Centers for Disease Control and Prevention (CDC) recommends the influenza vaccine every year for the following groups:

  • all persons who want to reduce the risk of becoming ill with influenza or of transmitting influenza to others;
  • all children aged 6 months through 18 years of age;
  • all persons aged 50 years and older;
  • women who will be pregnant during the influenza season;
  • adults and children who have chronic pulmonary (including asthma), cardiovascular (except hypertension), renal, hepatic, hematological or metabolic disorders (including diabetes mellitus);
  • adults and children who have immunosuppression (including immunosuppression caused by medications or by human immunodeficiency virus;
  • adults and children who have any condition (e.g., cognitive dysfunction, spinal cord injuries, seizure disorders, or other neuromuscular disorders) that can compromise respiratory function or the handling of respiratory secretions or that can increase the risk for aspiration;
  • residents of nursing homes and other chronic-care facilities;
  • health-care personnel;
  • healthy household contacts (including children) and caregivers of children aged <5 years and adults aged 50 years and older, with particular emphasis on vaccinating contacts of children aged <6 months; and
  • healthy household contacts (including children) and caregivers of persons with medical conditions that put them at higher risk for severe complications from influenza.

Individuals with severe hypersensitivity to eggs or those who have had a previous vaccine-associated allergic reaction should avoid immunization.

Vaccine Effectiveness and Safety

There are two types of influenza vaccine available. Both are effective in preventing influenza infection.

Inactivated vaccine (injected): The inactivated influenza vaccine can be given to anyone 6 months of age and older. It is given as a shot in the upper arm or in the thigh. There may be some mild soreness, redness or swelling at the injection site, which may last one to two days. Other possible mild side effects include a headache and low-grade fever for a day after vaccination.

Live vaccine (nasal spray): The live vaccine can only be given to healthy* persons 2 to 49 years of age†. It is given as a spray in each nostril. It cannot be used in persons with certain chronic medical conditions (such as asthma, diabetes and heart disease), pregnant women or those who are in close contact with people who have weakened immune systems. There may be a runny nose, headache, low-grade fever, sore throat, tiredness or cough after vaccination.

Key Facts about Influenza and its Prevention

  • Influenza ("the flu") is a severe respiratory infection with affects millions of people each year. Other mild respiratory illnesses and even intestinal syndromes are often mistakenly referred to as the flu.
  • Influenza can be prevented with safe, effective vaccines.
  • Because influenza viruses can change from year to year and because protection from the vaccine does not last more than one year, an influenza vaccination is necessary each year.
  • To protect children younger than 6 months of age from influenza and its complications, household members and other close contacts of these infants should be vaccinated.
  • The majority of influenza-related deaths are in older persons, but deaths do occur in children. There is an average of 92 deaths each year in children under than 5 years of age.
  • Influenza can worsen other medical conditions like heart disease, lung disease and diabetes, and it can lead to pneumonia.
  • Influenza vaccine can prevent 50 to 60 percent of hospitalizations and 80 percent of deaths from influenza-related complications among elderly persons in nursing homes.
  • Influenza vaccine will not protect people from other respiratory infections, such as colds or bronchitis.

July 2008

* "Healthy" indicates persons who do not have an underlying medical condition that predisposes them to influenza complications.

&dagger On October 24, 2007 CDC's Advisory Committee on Immunization Practices (ACIP) recommended expanding the use of the nasal influenza vaccine LAIV (FluMist®) to include healthy children ages 2-4 years old (24-59 months old) without a history of asthma or recurrent wheezing. The vaccine continues to be recommended for healthy persons ages 5-49 years who are not pregnant.