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Questions & Answers about the 2008-2009 Flu Season

2008-09 Influenza (Flu) Season

What sort of flu season is expected this year?

Flu seasons are unpredictable in a number of ways. Although epidemics of flu happen every year, the beginning, severity, and length of the epidemic depends on many factors, including the different types and strains of influenza viruses circulating and whether the viruses in the vaccine match flu viruses that are circulating.

CDC recommends a yearly flu vaccine as the first and most important step in protecting against this serious disease. While there are many different flu viruses, the flu vaccine is designed to protect against the three main flu strains that research indicates will cause the most illness during the flu season. The vaccine can protect you from getting sick from these three viruses or it can make your illness milder if you get a different flu virus.

Flu activity typically does not reach its peak in the U.S. until January or February. Getting the flu vaccine soon after it becomes available each year is always a good idea, and the protection you get from vaccination will last throughout the flu season. However, flu activity can occur as late as May so getting a vaccine later in the season, including in December, January or even later, and even if flu activity has already started in your area, can still offer protection in most years.

Will new strains of flu circulate this season?

Flu viruses are constantly changing so it's typical for new strains of flu viruses to appear each year. For more information about how flu viruses change, visit “How the Flu Virus Can Change.”

How effective is the flu vaccine?

The effectiveness of the vaccine can vary and depends in part on the match between the viruses in the vaccine and flu viruses that are circulating in the community. If these are closely matched, vaccine effectiveness (VE) is higher. If they are not closely matched, VE can be reduced. During well-matched years, clinical trials have shown VE between 70% and 90% among healthy adults.

For more information about vaccine effectiveness, visit “How Well Does the Seasonal Flu Vaccine Work?”

What other factors can influence which viruses are chosen to go into the vaccine?

It’s not possible to predict with certainty which flu viruses will predominate during a given season or what the severity, timing, or duration of a flu season will be. Flu viruses are constantly changing (called drift) – they can change from one season to the next or they can even change within the course of one flu season. Experts must pick which viruses to include in the vaccine many months in advance in order for vaccine to be produced and delivered on time. (For more information about the vaccine virus selection process visit “Selecting the Viruses in the Influenza (Flu) Vaccine.”) Because of these factors, there is always the possibility of a less than optimal match between circulating viruses and the viruses in the vaccine.

Over the course of a flu season CDC studies samples of flu viruses circulating during that season to evaluate how close a match there is between viruses in the vaccine and circulating viruses. In addition, CDC conducts vaccine effectiveness studies to determine how well the vaccine protects against illness. However, it’s important to remember that even during seasons when the vaccine is not optimally matched to predominant circulating viruses, CDC and other experts continue to recommend flu vaccine as the best way to protect against the flu.

Can the vaccine provide protection even if the vaccine is not a “good” match?

Yes, antibodies made in response to vaccination with one strain of flu viruses can provide protection against different, but related strains. A less than ideal match may result in reduced vaccine effectiveness against the variant viruses, but it can still provide enough protection to prevent or lessen illness severity and prevent flu-related complications. In addition, it's important to remember that the flu vaccine contains three virus strains so that even when there is a less than ideal match or lower effectiveness against one strain, the vaccine may protect against the other two viruses. For these reasons, even during seasons when there is a less than ideal match, CDC continues to recommend flu vaccination. This is particularly important for people at high risk for serious flu complications and their close contacts.

In what years was there a good match between the vaccine and the circulating viruses?

In recent years the match between the vaccine viruses and those identified during the flu season has usually been good. In 16 of the last 20 U.S. influenza seasons the viruses in the influenza vaccine have been well matched to the predominant circulating viruses. Since 1988, there has only been one season (1997-98) when there was very low cross-reaction between the viruses in the vaccine and the predominate circulating virus and three seasons (1992-93, 2003-04, and 2007-08) when there was low cross-reaction.

What is CDC doing to monitor vaccine effectiveness for the 2008-09 season?

CDC carries out and collaborates with other partners within and outside CDC to assess the effectiveness of flu vaccines. During the 2008-09 season, CDC is planning multiple studies on the effectiveness of influenza vaccine. These studies will measure vaccine effectiveness in preventing laboratory confirmed influenza in older people and in children.

What determines the severity of a flu season?

The overall health impact (e.g., infections, hospitalizations and deaths) of a flu season varies from year to year. Based on available data from U.S. influenza surveillance systems monitored and reported by CDC, the severity of a flu season can be judged according to a variety of criteria, including:

A season's severity is determined by comparing these measures with previous seasons.

What is CDC doing to monitor antiviral resistance in the United States during the 2008-09 season?

Antiviral resistance means that a virus has changed in such a way that the antiviral drug is less effective in treating or preventing illnesses caused by the virus. Samples of viruses collected from around the United States and worldwide are studied to determine if they are resistant to any of the four FDA-approved influenza antiviral drugs.

CDC routinely collects viruses through a domestic and global surveillance system to monitor for changes in influenza viruses. CDC will continue ongoing surveillance and testing of influenza viruses. Additionally, CDC is working with the state public health departments and the World Health Organization to collect additional information on antiviral resistance in the United States and worldwide. The information collected will assist in making informed public health policy recommendations.

What actions can I take to protect myself and my family against the flu this season?

CDC recommends a yearly flu vaccine as the first and most important step in protecting against this serious disease. While there are many different flu viruses, the flu vaccine protects against the three main flu strains that research indicates will cause the most illness during the flu season. The vaccine can protect you from getting sick from these three viruses or it can make your illness milder if you get a different flu virus.

If you do get the flu, antiviral drugs are an important treatment option. Antiviral drugs are prescription medicines (pills, liquid or an inhaler) that fight against the flu by keeping flu viruses from reproducing in your body. Antiviral drugs can make your illness milder and make you feel better faster. They may also prevent serious flu complications. This could be especially important for people at high risk. For treatment, antiviral drugs work best if started soon after getting sick (within 2 days of symptoms). Two FDA-approved influenza antiviral agents are recommended for use in the United States to treat or prevent flu during the 2008-09 influenza season: oseltamivir and zanamivir.

In addition, you can take everyday preventive steps like frequent hand washing to decrease your chances of getting the flu. If you are sick with flu, reduce your contact with others and cover your cough to help keep germs from spreading.

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