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Press Release

For Immediate Release: Juanuary 7, 2000
Contact: CDC Media Relations (404) 639-3286

Flu Season 1999-2000: Questions and Answers

What is influenza (flu)?

Influenza, commonly called "the flu," is caused by the influenza virus, which infects the respiratory tract. The virus is typically spread from person-to-person when an infected person coughs or sneezes the virus into the air. Compared with other viral respiratory infections such as the common cold, influenza infection can cause severe illness and also precipitate serious and life-threatening complications in all age groups.

Typical clinical features of flu include fever, dry cough, sore throat, runny or stuffy nose, headache, muscle aches, and extreme fatigue. Children may experience gastrointestinal symptoms such as 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 microorganisms and is not related to true influenza. Some of the medical complications precipitated by flu include bacterial pneumonia, dehydration, and worsening of underlying chronic conditions, such as congestive heart failure and asthma. Complications occur most often in persons who are particularly vulnerable, such as the elderly and persons of any age with certain chronic health conditions.

Is the United States undergoing a flu epidemic?

In the United States, flu outbreaks typically occur during the winter months from late December through March, leading to increased numbers of people with respiratory illness ("the flu"). The start, peak period, duration and total health impact (i.e., hospitalizations and deaths) of the flu season can vary considerably from year-to-year. CDC monitors this activity and the associated influenza viruses and provides summaries of national influenza surveillance results each week during October through April (with the exception of the last week of the year during the holidays). These weekly summaries are available online

Do other respiratory viruses circulate during the influenza season?

In addition to influenza viruses, it is important to note that other respiratory pathogens also frequently circulate during the same time period as influenza viruses and can cause similar respiratory illness. These pathogens include viruses such as respiratory syncytial virus, which is the most common cause of severe respiratory illness in young children.

Is there a new flu virus circulating this season?

In the United States, the predominant influenza virus circulating this winter (1999-2000) has been the influenza A/Sydney/5/97-like (H3N2) virus. This virus has been the predominant influenza virus since the1997-98 flu season. Because influenza viruses change over time through mutations (a process called "drift"), CDC works closely with surveillance laboratories to detect emerging variant viruses. S o far this year, the 1999-2000 flu vaccine is well matched to the viruses circulating in the United States. Surveillance for changes in circulating viruses continues year round.

What can people do to protect themselves against flu?

By far, the single most important preventive measure is for individuals, especially persons at high risk for serious complications, to get vaccinated in the fall.

Who should get a flu shot?

The most important major group of persons who should receive flu vaccine are those who are at high risk for developing serious complications from flu. These individuals include people aged 65 years or older and people of any age with chronic diseases of the heart, lung, or kidneys, diabetes, immunosuppression, or severe forms of anemia. Other groups for whom vaccine is specifically recommended are residents of nursing homes and other chronic-care facilities housing patients of any age with chronic medical conditions, and children and teenagers who are receiving long-term aspirin therapy and who may therefore be at risk for developing Reye syndrome after influenza virus infection. The second major group of persons who should be vaccinated against flu are those who are in close or frequent contact with anyone in the high-risk groups defined above. These people include healthcare personnel and volunteers who work with high-risk patients and people who live in a household with a high-risk person. The primary reason for vaccinating this group is to decrease the risk of passing influenza infection to persons at high risk for serious complications. Finally, anyone who wishes to decreases their chances of developing flu can receive the flu vaccine.

Is the flu vaccine effective immediately after a person receives the shot?

It takes about 1 to 2 weeks after vaccination for antibody against influenza to develop and provide protection.

How well does the flu vaccine work?

The effectiveness of flu vaccine in protecting individuals against illness or serious complications of flu depends on primarily: 1) the age and health status of the person receiving the vaccine and 2) the similarity or "match" between the virus strains in the vaccine and those in circulation. This year's vaccine is well-matched to the virus strains that have been circulating during the 1999-2000 flu season. When the "match" between vaccine and circulating strains is close, flu vaccine prevents illness in approximately 70%-90% of healthy persons younger than age 65 years. Among elderly persons living outside chronic-care facilities (such as nursing homes), flu vaccine is 30%-70% effective in preventing hospitalization for pneumonia and influenza. Among elderly nursing home residents, flu vaccine is most effective in preventing severe illness, secondary complications, and deaths. In this population, the vaccine can be 50%-60% effective in preventing hospitalization or pneumonia and 80% effective in preventing death from influenza infection.

Should individuals still get vaccinated?

The optimal time for organized vaccination campaigns to immunize persons in high-risk groups against flu is from October through mid-November. However, provided that vaccine is still available, flu vaccine should be administered to high-risk individuals who have not yet been vaccinated even after mid-November. As long as flu vaccine is available, and flu activity is continuing, it is appropriate for persons at high-risk for serious complications from influenza to receive flu vaccine.

How are the viruses for flu vaccine selected?

Each year, influenza viruses are collected by many laboratories throughout the world, including the United States. Some of these viruses are forwarded for further analysis to one of four World Health Organization (WHO) reference laboratories, one of which is at the Centers for Disease Control and Prevention in Atlanta. This information, along with epidemiologic information on flu activity, is summarized and presented to an advisory committee of the Food and Drug Administration and at a WHO meeting. These meetings result in the selection of three viruses (two different subtypes of influenza A viruses and one influenza B virus) to go into flu vaccine strains for the following fall and winter. Usually one or two virus strains change in the vaccine each year.

What are some of the most common misconceptions about flu?

There are several common misconceptions about flu including the following:

  • Misconception #1. 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.
  • Misconception #2 - 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.
  • Misconception #3 - Flu vaccine is not very effective.
    When the vaccine viruses and circulating viruses are well matched, vaccine can be very effective. There are several different reasons for this belief. 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, in the elderly vaccine is effective in reducing hospitalizations and death from flu related causes.

What are the adverse effects of flu vaccine?

By far, the most common side effect of flu vaccine is local arm soreness and swelling . This is usually mild and does not prevent most persons from working. The most serious side effect that can occur after flu vaccination are uncommon and include severe allergic reactions, particularly in people who have a severe allergy to eggs (the vaccine viruses are grown in eggs). In addition, the 1976 swine flu vaccine was associated with Guillain Barre Syndrome (GBS). Evidence for a causal relationship of GBS with subsequent vaccines prepared from other virus strains is less clear. Further discussion can be found in "Prevention and Control of Influenza: Recommendations of the Advisory Committee on Immunization Practices (ACIP)." The document is available online at

Why must flu vaccine be given every year?

Influenza viruses are continually changing, which is why the viruses in the vaccine must be updated often. Vaccine for one year may not cover viruses circulating in the next season. Another reason is that antibody resulting from flu vaccine declines over time, and antibody levels from one year can be too low to provide protection for the next year.

Is there a shortage of flu vaccine this year?

This year, over 80 million doses of flu vaccine were produced for distribution in the United States . Typically, most of the flu vaccine supply is distributed and administered during the fall and by late December and January, the amount of vaccine that is available is limited. Individuals who want to be vaccinated should contact their regular health care provider to inquire about the availability of flu vaccine. Healthcare providers can contact either a local vaccine distributor or one of the manufacturers directly to inquire about the availability of vaccine for purchase.

If I am exposed to a person with flu and become infected, how soon will I get sick?

The typical time from when a person is exposed to influenza virus to when symptoms begin is about 1-4 days.

How long is a person with flu contagious?

This depends on the age of the person, but adults usually can shed influenza virus for up to 3-5 days after they first develop symptoms. Some children may shed virus for longer than a week.

How many people get sick with the flu each year? How many die?

Every flu season is unique but on average it is estimated that approximately 10% to 20% of the general U.S. population develops flu, more than 110,000 persons are hospitalized from flu-related complications, and approximately 20,000 Americans die from the complications of flu.

Will the new antivirals cure the flu?

Four different antiviral drugs (amantadine, rimantadine, zanamivir and oseltamivir) have been approved for treating influenza illness. All four drugs can reduce the duration of moderate or severe symptoms of influenza by about one day if started within 48 hours of the start of symptoms. The four drugs differ in terms of side effects. In some patients, amantadine (Symmetrel, others) can cause nervousness, difficulty concentrating, or lightheadedness. Rimantadine (Flumadine) can also cause similar types of side effects, but less frequently. Caution is advised if zanamivir (Relenza) is used by people who have asthma or chronic obstructive pulmonary disease (COPD), because some people with asthma or COPD may have bronchospasm or difficulty breathing after using zanamivir. Oseltamivir (Tamiflu)can cause nausea and vomiting in some people. All of these drugs are prescription drugs and a physician should be consulted before the drugs are used. These drugs are effective against influenza viruses, but they are not effective for illnesses caused by other viruses or bacteria that can have symptoms similar to influenza. These drugs are also not effective for treating bacterial infections that can occur as complications of influenza.

Will antiviral medications prevent the flu?

Two of the antiviral drugs (amantadine and rimantadine) have been approved for prophylaxis (prevention) of influenza infection. These drugs are not, however, a substitute for influenza vaccination. Both of these drugs are prescription drugs and a physician should be consulted before the drugs are used for preventing influenza.

How can a person tell if they have the flu, a cold, or some other respiratory infection?

It is not always possible to distinguish influenza illness from other viral or bacterial respiratory illnesses on the basis of symptoms alone. A test must be performed to confirm that an illness is influenza. In addition, a doctor's evaluation may be needed to determine whether a person has another infection complicating influenza.


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