Questions & Answers

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What causes influenza?
Viruses cause influenza. There are two basic types, A and B. Their genetic material differentiates them.

Influenza A can cause moderate to severe illness in all age groups and infects humans and other animals. Influenza B causes milder disease and affects only humans, primarily children.

Subtypes of the type A influenza virus are identified by two antigens (proteins involved in the immune reaction) on the surface of the virus. These antigens can change, or mutate, over time. When a "shift" (major change) or a "drift" (minor change) occurs, a new influenza virus is born and an epidemic is likely among the unprotected population.

How does influenza spread?
Influenza is transmitted through the air from the respiratory tract of an infected person. It can also be transmitted by direct contact with respiratory droplets.

How long does it take to develop symptoms of influenza after being exposed?
The incubation period of influenza is usually two days but can range from one to five days.

What are the symptoms of influenza?
Typical influenza disease is characterized by abrupt onset of fever, aching muscles, sore throat, and non-productive cough. Additional symptoms may include runny nose, headache, a burning sensation in the chest, and eye pain and sensitivity to light. Typical influenza disease does not occur in every infected person. Someone who has been previously exposed to similar virus strains (through natural infection or vaccination) is less likely to develop serious clinical illness.

How serious is influenza?
Although many people think of influenza as a type of cold, it is really a specific and serious disease. Disease complications and death are more common among young children, the elderly, and those with chronic illnesses. In the United States, the number of influenza-associated deaths has increased since 1990. This increase is due in part to the substantial increase in the number of persons age 65 years or older, who are at increased risk for death from influenza complications. An average of 36,000 influenza-associated pulmonary and circulatory deaths per season occurred during 1990-1999, compared to 19,000 such deaths per influenza season during 1976-1990.

Influenza viruses cause disease among persons of all ages. Rates of infection are highest among children, but the risks for complications, hospitalizations, and deaths from influenza are higher among persons age 65 years or older, young children, and persons of any age who have medical conditions that place them at increased risk for complications from influenza. Case reports and several epidemiologic studies also indicate that pregnancy can increase the risk for serious medical complications of influenza.

In nursing homes, up to 60% of residents may be infected, with up to a 30% fatality rate in the infected. Risk for influenza-associated death is highest among the oldest elderly: persons age 85 years and older are 16 times more likely to die from an influenza-associated illness than persons aged 65-69 years.

Children age two years and younger have hospitalization rates second only to people age 65 years and older. Children younger than age one year are the most likely to be hospitalized. Influenza-associated deaths are uncommon among children but represent a substantial proportion of vaccine-preventable deaths. An estimated annual average of 92 influenza-related deaths occurred among children age 5 years or younger during the 1990s, compared with 32,651 deaths among adults age 65 years or older.

The cost of a severe epidemic has been estimated at $12 billion. Occasionally, major epidemics occur on an international scale. This is known as a pandemic. The first recording of such an event was in 1580, and at least seven international epidemics have occurred in the nineteenth and twentieth centuries. The "Spanish flu" epidemic of 1918-1919 caused an estimated 21 million deaths worldwide, including more than 500,000 Americans.

How many people in the United States are hospitalized with influenza in a typical year?
A study conducted by CDC and published in the Journal of American Medical Association (JAMA) on September 15, 2004, provided new information on the number of people in the United States who are hospitalized from influenza-related complications each year. The study was based on records from 1979 to 2001 from about 500 hospitals across the United States. The study concluded that, on average, more than 200,000 people in the United States are hospitalized each year for respiratory and heart-related illnesses associated with influenza virus infections.

What are possible complications from influenza?
The most frequent complication of influenza is bacterial pneumonia. Viral pneumonia is a less common complication but has a high fatality rate. Other complications include inflammation of the heart and worsening of such pulmonary diseases as bronchitis.

Reye's syndrome is a complication that occurs almost exclusively in children--patients suffer from severe vomiting and confusion, which may progress to coma because of swelling of the brain. To decrease the chance of developing Reye's syndrome, infants, children, and teenagers should not be given aspirin for fever reduction or pain relief.

What is the best way to prevent influenza?
The best way to prevent influenza is with annual vaccination.

Is there an alternative to vaccination in preventing influenza?
Vaccination is the principal means of preventing influenza and its complications. Here are some additional steps that may help prevent the spread of respiratory illnesses like influenza:

1. Cover your nose and mouth with your sleeve or a tissue when you cough or sneeze--throw the tissue away after you use it.
2. Wash your hands often with soap and water, especially after you cough or sneeze. If you are not near water, use an alcohol-based hand cleaner.
3. Stay away as much as you can from people who are sick.
4. If you get influenza, stay home from work or school. If you are sick, don't go near other people to avoid infecting them.
5. Try not to touch your eyes, nose, or mouth. Germs often spread this way.

There are four antiviral agents approved for preventing or treating influenza in selected patients. Only two, oseltamivir and zanamavir, will offer protection against both A and B viruses; the other two, amantadine and rimantadine, protect only against the A viruses. Their use is generally limited to situations where an outbreak is underway and immediate protection of vulnerable, unvaccinated persons is critical (e.g., nursing home residents) or in persons who are expected to have an inadequate antibody response to the vaccine (e.g., persons infected with HIV) or who could not otherwise be vaccinated (e.g., persons with severe egg allergies). Antiviral agents are not a substitute for vaccination. (Note: Recent evidence indicates that a high proportion of currently circulating influenza A viruses in the United States have developed resistance to amantadine and rimantadine so that these two antivirals cannot be used during the 2007-08 influenza season.)

If I contract influenza, what should I do?
Call your healthcare provider to discuss your particular situation. You will need to get plenty of rest and to drink a lot of liquids. You can take medications to relieve the symptoms of influenza (but never give aspirin to children or teenagers who have influenza-like symptoms, particularly fever). If you are at high risk from complications of influenza, you should consult your healthcare provider immediately if you develop influenza-like symptoms. Those at high risk for complications include people 65 years or older, people with chronic medical conditions, pregnant women, and young children. Your doctor may recommend use of an antiviral medication to help treat influenza.

When is a person with influenza contagious?
A person is most likely to pass on the virus during the period beginning one to two days before the onset of symptoms and ending four to five days after the onset.

Why can't we eradicate influenza as we are doing with some other vaccine-preventable diseases (e.g., polio)?
It is difficult to completely eliminate influenza for several reasons:

1. Influenza viruses mutate frequently, making it very difficult to provide one influenza vaccination that will protect an individual for life.
2. Each year's influenza vaccine is made up of three strains of the virus, based on an educated guess of which viruses will be most active during the upcoming influenza season. Occasionally, this projection may be wrong, and that year's vaccine will be less effective.
3. Influenza vaccine is not completely effective at preventing infection, especially with older individuals (although it does protect them from serious complications and death).
4. No attempt is made to vaccinate the entire population. Instead, influenza vaccine is mainly recommended for certain groups such as people over 50, healthcare workers, people with chronic underlying illnesses, and others. Most recently the vaccine was recommended for use in infants and children age 6-59 months.

Can you get influenza more than once?
Yes. Influenza viruses change frequently and infection with one strain does not provide protection against all strains.

Questions and answers about influenza vaccine

Reviewed by the Centers for Disease Control and Prevention, October 2007