Questions & Answers

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What causes seasonal influenza?
Viruses cause influenza. There are two basic types, A and B, which can cause clinical illness in humans. 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 seasonal 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 seasonal influenza after being exposed?
The incubation period of seasonal influenza is usually two days but can range from one to four days.

What are the symptoms of seasonal influenza?
Typical seasonal 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 seasonal 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 seasonal influenza?
Although many people think of seasonal influenza as the “flu” or just a common cold, it is really a specific and serious respiratory disease that can result in hospitalization and death. In the United States, the number of seasonal 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 seasonal 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 disease can occur among persons of all ages; however, the risks for complications, hospitalizations, and deaths are higher among persons age 65 years or older, young children, and persons of any age who have certain medical conditions. Case reports and epidemiologic studies also indicate that pregnancy can increase the risk for serious medical complications from influenza.

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

Hospitalization from influenza-related complications is also high among children age 24 months and younger - comparable to rates for persons age 65 and older. There were 153 laboratory-confirmed seasonal influenza-related pediatric deaths reported during the 2003-04 influenza season. In the following four influenza seasons, the annual number of pediatric deaths reported to CDC ranged from 44 during the 2004-05 season to 84 during the 2007-08 season.

How is a pandemic different from an epidemic of influenza?
Occasionally, major influenza epidemics expand to a pandemic. The first recording of such an event was in 1580, and at least seven international pandemics 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. On June 11, 2009, the World Health Organization (WHO) officially declared that the spread of the novel influenza A (H1N1) virus had become a pandemic. The novel virus, which first appeared in Mexico during the spring of 2009, had demonstrated person-to-person transmission on multiple continents, meeting the definition of a pandemic.

How many people in the United States are hospitalized with seasonal 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 seasonal 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 cardiac-related illnesses associated with seasonal influenza virus infections.

What are possible complications from seasonal influenza?
The most frequent complication of seasonal 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 pulmonary diseases (e.g., 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 seasonal influenza?
The best way to prevent seasonal 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 for at least 24 hours after the fever has ended. 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.

What other drugs are available to prevent or treat seasonal influenza?
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 and researchers are watching for additional antiviral resistance to any of these four agents that might develop in the future.)

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 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 of developing complications from influenza, you should consult your healthcare provider immediately if you develop influenza-like symptoms. For purposes of treatment and prevention (chemoprophylaxis), antiviral medicines are prioritized for persons at high risk for influenza-related complications, such as people 65 years or older, people with chronic medical conditions, pregnant women, and young children.

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.

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 2009