2009-10 Influenza (Flu) Season - Questions & Answers about the 2009-2010 Flu Season

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What sort of flu season is expected this year?

Flu seasons are unpredictable in a number of ways, including the timing of the beginning, severity, and length of the flu season.

This flu season (2009-2010), there are more uncertainties than usual because of the emergence of a new 2009 H1N1 influenza virus (previously called "novel H1N1" or "swine flu") that has caused the first influenza pandemic (global outbreak of disease) in more than 40 years.

Severity is uncertain. Many people do not have immune protection against this new and very different 2009 H1N1 virus, which has spread worldwide quickly and has been declared a pandemic by the World Health Organization (WHO). Scientists believe the 2009 H1N1 virus - along with regular seasonal viruses - will cause illness, hospital stays, and deaths this flu season in the United States. There is concern that the 2009 H1N1 virus may cause the season to be worse than a regular flu season - with a lot more people getting sick, being hospitalized and dying than during a regular flu seasonal.

Timing is uncertain. In past years, seasonal flu activity typically did not reach its peak in the U.S. until January or February, but flu activity has occurred as late as May. However, the 2009 H1N1 virus caused illness, hospitalizations, and deaths in the U.S. during the summer months when influenza is very uncommon. So it is not known when flu activity will increase, when it will be most intense (peak), what viruses will circulate, or how long the season might last.

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:

• The level of reported activity within each state;
• The number and proportion of flu laboratory tests that are positive;
• The proportion of visits to physicians for flu-like illness;
• The proportion of all deaths that are caused by pneumonia and flu;
• The number of flu-associated deaths among children; and
• The flu-associated hospitalization rate among children and adults.

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

The effectiveness of flu vaccines can vary and depends in part on the match between the viruses in the vaccine and the 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 seasonal flu vaccine effectiveness, visit "How Well Does the Seasonal Flu Vaccine Work?"

It's not possible to predict with certainty which seasonal 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 seasonal flu 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 flu viruses and the viruses in the seasonal flu vaccine. This season, it's likely that the 2009 H1N1 virus will circulate in the United States. A seasonal vaccine will not protect you against 2009 H1N1, but a vaccine against 2009 H1N1 is being produced.

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.

Yes, antibodies made in response to vaccination with one flu virus can provide protection against related viruses. This can result in reduced vaccine effectiveness against the related viruses, but it can still provide enough protection to prevent or lessen illness severity and prevent flu-related complications. (However, a flu vaccine is not expected to offer cross-protection against viruses that are very different genetically from those in the vaccine.) In addition, it's important to remember that the seasonal flu vaccine contains three viruses so that even when there is a less than ideal match or lower effectiveness against one virus, 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 seasonal flu vaccination. This is particularly important for people at high risk for serious flu complications and for their close contacts.

The seasonal vaccine is not expected to protect against the 2009 H1N1 virus because it is very different genetically from the seasonal H1N1 virus that is in this season's vaccine. That is why the government and manufacturers are producing a separate 2009 H1N1 vaccine designed to protect against the 2009 H1N1 virus.

CDC carries out and collaborates with outside partners to assess the effectiveness of seasonal flu vaccines. During the 2009-10 season, CDC is planning multiple studies on the effectiveness of the seasonal influenza vaccine and the 2009 H1N1 vaccine. These studies will measure vaccine effectiveness in preventing laboratory-confirmed seasonal and 2009 H1N1 influenza infections in children, pregnant women, health care workers, and adults.

Antiviral resistance means that a virus has changed in such a way that antiviral drugs are 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 seasonal influenza viruses and 2009 H1N1 influenza viruses to check for antiviral resistance. Recently, CDC has implemented enhanced surveillance across the United States to detect oseltamivir resistance in 2009 H1N1 viruses. CDC also 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.

CDC recommends a yearly seasonal flu vaccine as the first and most important step in protecting against seasonal flu. While there are many different flu viruses, the seasonal flu vaccine protects against the three main seasonal flu strains that research indicates will cause the most illness during the flu season. The seasonal flu vaccine can protect you from getting sick from these three viruses or it can make your illness milder if you get a related flu virus. The seasonal flu vaccine will not provide protection against the new H1N1 influenza. However a 2009 H1N1 vaccine is currently in production and is expected to be ready for the public in the fall. The 2009 H1N1 vaccine is not intended to replace the seasonal flu vaccine - it is intended to be used along-side seasonal flu vaccine.

In addition, there are everyday actions that can help prevent the spread of germs that cause respiratory illnesses like influenza.

Take these everyday steps to protect your health:

• Cover your nose and mouth with a tissue when you cough or sneeze. Throw the tissue in the trash after you use it.
• Wash your hands often with soap and water, especially after you cough or sneeze. If soap and water are not available, an alcohol-based hand rub can be used.
• Avoid touching your eyes, nose and mouth. Germs spread this way.
• Try to avoid close contact with sick people.
• If you are sick with flu-like illness, CDC recommends that you stay home for at least 24 hours after your fever is gone except to get medical care or for other necessities. (Your fever should be gone without the use of a fever-reducing medicine.) Keep away from others as much as possible to keep from making others sick.

Antiviral drugs are prescription medicines (pills, liquid or an inhaled powder) that fight against the flu, including the new H1N1 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. The priority use for influenza antiviral drugs this flu season is to treat people with more severe illness, those that are hospitalized, and to treat people who are sick who also are at increased risk of severe illness, including pregnant women, young children, people 65 years of age and older, and people with chronic health conditions like asthma, diabetes and other metabolic diseases, heart or lung disease, kidney disease, weakened immune systems, and persons with neurologic or neuromuscular disease. For treatment, antiviral drugs work best if started within the first 2 days of symptoms.

If you become ill with influenza-like symptoms this flu season you should stay home and avoid contact with other people, except to seek medical care. If you have severe illness or you are at higher risk for flu complications, contact your health care provider or seek medical care. Medical conditions that can place you at higher risk of serious flu-related complications if you become ill include asthma or other lung problems, diabetes, weakened immune systems, kidney disease, heart disease, neurological and neuromuscular disorders and pregnancy.

While few people over the age of 65 have been infected with this new virus, if you are older than 65 and become ill, you are at higher risk of developing flu-related complications. Children younger than 5 years of age are also at high risk of serious complications if they get the flu.

If you have one of these medical conditions or are 65 or older or younger than 5 and develop flu-like symptoms including fever or chills and cough or sore throat, contact your health care provider so they may determine whether flu testing or treatment is needed.

For information about 2009 H1N1 flu, visit http://www.cdc.gov/h1n1flu/