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National Vaccine Program Office

Ongoing Influenza Defense Tactics

Fighting the flu in the U.S. is a yearly battle that requires the combined resources of the Department of Health and Human Services, the World Health Organization (WHO), vaccine and drug companies, State and local health authorities, and the medical community.

Early detection of changes in influenza viruses and rapid development of effective vaccines are the keys to defending against influenza each year and responding to the possibility of a pandemic. The cycle of surveillance and vaccine formulation is a never-ending process.

Ongoing Surveillance

The first line of defense against influenza is a worldwide surveillance system coordinated by WHO. This system makes it possible for changes in circulating influenza viruses and the emergence of novel influenza A viruses to be detected as soon as possible.

The task of identifying circulating strains of influenza--whether known or novel--is done by a worldwide network of 110 National Influenza Centers and many other WHO laboratories in 83 countries. WHO Collaborating Reference Centers for Influenza in London, Atlanta, Melbourne, and Tokyo coordinate the system and intensively analyze samples of virus isolated and collected by approximately 180 laboratories.

Each year, some influenza virus isolates from laboratories in the U.S. and overseas are sent to the Centers for Disease Control and Prevention (CDC) in Atlanta. Tests are done to determine the antigenic and molecular make-up of the viruses. CDC examines the viruses to determine which are the most important emerging influenza viruses, and their ability to cause outbreaks, and then provides this information at yearly meetings held by the Food and Drug Administration (FDA) and by WHO so it can be used to formulate vaccine for the next influenza season.

During January through March, WHO, The Food and Drug Administration (FDA), and CDC undertake the process of deciding which strains will be selected for vaccine production in the U.S.

In addition, the CDC actively monitors U.S. disease activity and deaths related to influenza between October and May of each year. This information is provided each week in influenza surveillance summaries.

B. Vaccine Development

The best method of preventing and reducing the severity of the flu is the timely development, distribution, and administration of influenza vaccine. The influenza vaccine used each year is an inactivated trivalent vaccine. This means that the flu vaccine contains three inactivated (or "killed") flu viruses that protect against three different strains of influenza virus (one influenza B and two influenza A strains). Because the current licensed vaccines are inactivated vaccines, flu vaccine cannot cause the flu � a common misperception. The effectiveness of the trivalent vaccine depends upon the "match" between strains of influenza that are circulating and the viruses in the vaccine. Although there is no guarantee that the strains picked for the vaccine will be the strains that go around during the following flu season, the match between vaccine strains and circulating strains is good about 90 percent of the time.

The vaccine strain selection process requires surveillance information collected year-round. In late January of each year, the FDA's Vaccines and Related Biological Products Advisory Committee (VRBPAC) reviews worldwide surveillance data. The Committee usually makes an initial recommendation about at least one of the three strains to be included in the vaccine. By mid-February, the WHO completes its review and makes recommendations for the Northern Hemisphere vaccine. The WHO repeats this process in September for Southern Hemisphere vaccine recommendations. In March, VRBPAC meets to finalize the recommendations for the U.S. influenza vaccine.

While the vaccine strain selection process is going on, the four influenza vaccine manufacturers licensed in the U.S. begin preparations for vaccine production. Because flu vaccine viruses are grown inside eggs, manufacturers must buy enough eggs to manufacture 80 million or more doses of vaccine. The FDA prepares the specific viral material for the manufacturers to use, in order to begin vaccine production. During the manufacturing process, the live viral ingredient is killed so that the vaccine will not cause people to become sick with the flu. As the manufacturers produce vaccine, FDA reviews safety data. The last steps of vaccine preparation include production and bottling of vaccine, distribution to vaccine providers, and administration to patients. All this must be done in time for vaccination campaigns to begin by late September.

Working closely with State and local health authorities, partners in the private sector, CDC, FDA, and vaccine manufacturers have built a successful program for vaccine delivery each year. CDC and its Advisory Committee on Immunization Practices (ACIP) issue recommendations each year for the prevention and control of influenza. ACIP strongly recommends influenza vaccine for any person, 6 months of age or older, who is at increased risk for complications of influenza. Groups at increased risk include persons 65 years of age and older; residents of nursing homes and other chronic-care facilities; adults and children with chronic lung, heart, metabolic, kidney, or immune system disorders; and women who will be in the 2nd or 3rd trimester of pregnancy during the influenza season. Influenza vaccine also should be given to people who have close contact with high-risk persons, such as health care providers, family members of such persons, and others such as medical volunteers. The reason for vaccinating the close contacts is to prevent transmission of flu viruses to people who are at high risk for developing serious complications from flu. Influenza vaccine should also be administered to any person who wishes to reduce the likelihood of becoming ill with influenza.

C. Antiviral Drugs

In addition to vaccines, antiviral drugs are available for both the prevention and treatment of influenza. Currently, there are two classes of drugs--adamantines and neuraminidase inhibitors. The amantadines (amantadine and rimantadine), are approved for the treatment and prophylaxis of influenza A only. The neuraminidase inhibitors (zanamivir and oseltamivir) have been approved for the treatment of influenza A and B. Additionally, oseltamivir has been approved by the FDA for the prophylaxis of influenza. The FDAs Center for Drug Evaluation and Research provides a more complete discussion of approved antivirals on the following webpage:

To prevent the flu, antiviral drugs must be taken consistently before infection occurs. When used to reduce the impact of the flu for someone who is already infected, antiviral drugs must be taken within two days after flu symptoms start. It is important to know that antiviral drugs can have some potentially serious side effects.

In non-pandemic situations, antiviral drugs have been useful in helping to control outbreaks in settings such as nursing homes, where many people could become sick with flu and develop serious complications. In addition, antivirals can be useful in preventing influenza in certain individuals who have a weakened immune system and therefore, would not respond to the vaccine, or those who have a known allergic reaction to the vaccine. There are important differences among the influenza antiviral drugs, including age approved indications, side effects, and costs. A knowledgeable health care professional should be consulted when they are used.

During a pandemic, antiviral drugs are likely to play an important, but limited role. Guidelines are being developed to address how antiviral drugs should be used during a pandemic.

Next Section: Preparing for the Next Pandemic

Last revised: August 4, 2004


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