2009-10 Influenza (Flu) Season

Questions & Answers about the 2009–2010 Flu Season

What has this flu season been like?

Flu seasons are unpredictable in a number of ways, including when they begin, how severe they are, how long they last and which viruses will spread. There were more uncertainties than usual going into this flu season (2009-2010), because of the emergence of the 2009 H1N1 influenza virus (previously called "novel H1N1" or "swine flu"). This virus caused the first influenza pandemic (global outbreak of disease) in more than 40 years.

The United States experienced its first wave of 2009 H1N1 pandemic activity in the spring of 2009, followed by a second wave of 2009 H1N1 activity in the fall. The 2009-2010 flu season began very early, with 2009 H1N1 viruses predominating and causing high levels of flu activity much earlier in the year than during most regular flu seasons. In addition, the fall wave of 2009 H1N1 activity (like the spring) was relatively more severe among people younger than 65 than seasonal flu usually is and relatively less severe among people older than 65 than seasonal flu usually is. Activity so far this season peaked in October and then declined quickly to below baseline levels in January. The early rise in flu activity in October is in contrast to past influenza seasons. Influenza activity usually peaks in January, February or March. (See graph of peak influenza activity by month in the United States from 1976-2009.)

There are still uncertainties surrounding the rest of this flu season, including the possibility of the circulation of seasonal influenza viruses and ongoing circulation of 2009 H1N1 viruses. In past pandemics, flu activity has occurred in waves and ita??s possible that the United States could experience another wave of flu activity. In addition, sporadic cases of influenza may also be detected in the summer.

When did the flu season peak?

The weekly percentage of outpatient visits for influenza-like illness (ILI) peaked at the end of October at 7.7%, a level higher than the three previous influenza seasons, as reported by the U.S. Outpatient ILI Surveillance Network (ILINet). This percentage decreased to 1.8% at the beginning of January, 2010The number of states reporting widespread influenza activity dropped from 49 at the end of October to zero at the beginning of January 2010. While states are not reporting widespread influenza, most states are still reporting some influenza activity.

Is the flu season over?

Although influenza activity has declined recently, additional waves of influenza activity due to 2009 H1N1, seasonal influenza viruses, or both, may occur later this influenza season. Flu season can last as late as May. Even if the U.S. doesn"e;t experience a sharp increase in influenza activity during the remaining winter or spring (another "wave" of influenza), continued low level circulation of influenza viruses may continue during this time.

How is severity characterized?

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 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 assessing several of these measures and by comparing them with previous seasons.

How effective is the seasonal flu vaccine?

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?"

Are this seasona??s vaccines a good match for circulating viruses?

Flu viruses are constantly changing (called drift) a?? 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.

As of January 21, 2010, the predominant influenza virus in circulation is the 2009 H1N1 virus. More than 99% of all 2009 H1N1 viruses tested have been similar to the virus in the current 2009 H1N1 vaccine. Very few seasonal influenza viruses have been isolated and analyzed at CDC. For updated information about what viruses are circulating, visit FluView. CDC continues to recommend seasonal influenza vaccination to protect against seasonal influenza illness and vaccination of 2009 H1N1 vaccine to protect against the 2009 H1N1 pandemic virus.

How are vaccine match and vaccine effectiveness determined?

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.

Can the seasonal flu vaccine provide protection against other seasonal flu viruses even if the vaccine is not a "good" match?

Yes, antibodies made in response to vaccination with one flu virus can provide protection against related viruses. Vaccine effectiveness may be lower than if the vaccine and circulating strains are most alike but the vaccine 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"e;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. Even when one of the three components of the vaccine does not match exactly, 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.

Does the seasonal vaccine protect against 2009 H1N1 virus?

The seasonal vaccine does not protect against the 2009 H1N1 virus because it is very different genetically from the seasonal H1N1 virus that is in this season"e;s vaccine. A 2009 H1N1 vaccine is now widely available and CDC encourages vaccination for everyone as the best defense against 2009 H1N1.

What is CDC doing to monitor seasonal flu vaccine effectiveness for the 2009-10 season?

CDC carries out evaluations and collaborates with outside partners to assess the effectiveness of seasonal flu vaccines. During the 2009-10 season, CDC is conducting multiple studies on the effectiveness of the seasonal influenza vaccine and the vaccine against 2009 H1N1. 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.

What is CDC doing to monitor antiviral resistance in the United States during the 2009-10 season?

Antiviral resistance means that a virus has changed in such a way that antiviral drugs have become less effective in treating or preventing illnesses caused by the virus. Samples of viruses collected from around the United States and the world 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 is continuing 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. As of January 2009, almost all (98.7%) of the 2009 H1N1 influenza viruses tested for antiviral resistance at CDC have been susceptible to oseltamivir (Tamiflu??), and all of the viruses tested so far have been susceptible to zanamivir (Relenza??). 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.

What actions can I take to protect myself and my family against both seasonal flu and 2009 H1N1 flu this year?

Flu vaccination is the first and most important step in protecting against influenza viruses. Because the timing and spread of influenza viruses are unpredictable, CDC is continuing to recommend vaccination with seasonal influenza vaccine and 2009 H1N1 vaccine. At this point in the season, seasonal flu vaccine is limited. The seasonal flu vaccine will not provide protection against 2009 H1N1 influenza. However, supplies of 2009 H1N1 vaccines are ample and CDC is now encouraging everyone to get vaccinated, including people 65 years and older.

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.

What flu antiviral drugs does CDC recommend in the United States for the 2009-10 season?

Oseltamivir (trade name Tamiflu??) or zanamivir (trade name Relenza??) are recommended for treatment and prevention of flu during the 2009-10 season.

CDC has issued updated interim recommendations for the use of antivirals in the treatment of influenza for the 2009-10 season. This guidance is available at http://www.cdc.gov/h1n1flu/recommendations.htm.

What should I do if I get sick with the flu this flu season, including 2009 H1N1?

Antiviral drugs are prescription medicines (pills, liquid or an inhaled powder) that fight against the flu, including 2009 H1N1 flu, by keeping flu viruses from reproducing in your body. Ita??s very important that antiviral drugs be used early to treat flu in people who are very sick (for example people who are in the hospital) and people who are sick with flu and have a greater chance of getting serious flu complications (see box). Other people may also be treated with antiviral drugs by their doctor this season. Most healthy people with flu, however, do not need to be treated with antiviral drugs. Antiviral drugs can make your illness milder, make you feel better faster, and may also prevent serious flu complications. For treatment, antiviral drugs work best if started within the first 2 days after the onset 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.

If you are 65 or older and become ill, you are at higher risk of developing flu-related complications. Children younger than 5 years of age, but especially those younger than 2 years, are also at high risk of serious complications if they get the 2009 H1N1 flu.

If you have one of these medical conditions or are 65 or older or younger than 2 years of age 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/