Questions & Answers about the 2008-2009 Flu Season
2008-09 Influenza (Flu) Season
What sort of flu season are we having?
Since the beginning of the 2008-09 flu season, influenza activity has remained low in the United States but is now increasing. This is typical for this time of year and is similar to the past four influenza seasons. Of the few influenza viruses analyzed so far this season, most are related to the strains included in the 2008-09 influenza vaccine. Influenza A viruses, specifically Influenza A (H1) viruses, are circulating most commonly at this point in the season. However, it is still too early to tell which influenza virus types, sub-types and strains will predominate and cause the most illness this season.
CDC tracks flu activity year-round and weekly updates on flu activity are published in the “FluView” Weekly U.S. Influenza Surveillance Report.
When will flu activity peak this season?
It’s not possible to determine the peak of the influenza season until the flu season is over. Influenza activity typically does not reach its peak in the U.S. until January or February. During the past 26 flu seasons, the peak of the influenza season has occurred in November one season, December four seasons, January five seasons, February 12 seasons, and March four seasons.
As of January 15, flu activity in most of the country is still low but is increasing. Because influenza activity is just getting started, an influenza vaccine now can still protect you from the flu this season. Flu activity can occur as late as May and in any single season, more than one strain of influenza may circulate. So getting a vaccine later in the season can still offer protection, even if flu activity has already started in your area.
Are new strains of flu circulating this season?
Yes. Because influenza viruses change every year it’s common for new viruses to circulate every season. The flu season is just getting started right now, but so far this season, the majority of flu viruses studied by CDC have been related to the strains in the flu vaccine. For more information about how flu viruses change, visit “How the Flu Virus Can Change.”
What other factors can influence which viruses are chosen to go into the vaccine?
The effectiveness of the flu vaccine (ability to prevent influenza) can vary from season to season and also can vary depending on who is being vaccinated depending mainly on two things: 1) The characteristics of the person being vaccinated (such as their age and health) can affect vaccine effectiveness and 2) the relatedness or "match" between the influenza viruses in the vaccine and those spreading in the community can affect vaccine effectiveness. The closer the match, the more effective the vaccine is likely to be in preventing influenza. If the viruses in the vaccine are not related to circulating viruses, vaccine effectiveness can be lower. Although it’s early in the influenza season, most circulating influenza viruses so far are influenza A viruses, and these are related to the viruses in this season’s vaccine. Of circulating influenza B viruses, around one-third are related to the influenza B strain in this season's vaccine. At this time, it’s not possible to predict which viruses will predominate during the rest of the season, as every influenza season is different. Vaccination continues to be the primary tool recommended by CDC to prevent influenza infection.
For more information about vaccine effectiveness, visit “How Well Does the Seasonal Flu Vaccine Work?”
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. However, limited to no protection may be expected when the vaccine and circulating virus strains are so different as to be from different lineages, as is seen with the two lineages of influenza B viruses, e.g., the Yamagata and Victoria lineages. Among influenza A viruses, a less than ideal vaccine 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 with one or more strains in the vaccine, CDC continues to recommend flu vaccination. This is particularly important for people at high risk for serious flu complications and their close contacts.
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:
- The level of reported activity within each state;
- The 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.
A season's severity is determined by comparing these measures with previous seasons.
Where can I find information about antiviral resistance in the United States this season?
For more information about antiviral resistance in the United States during the 2008-09 flu season, please see Questions and Answers: Influenza Antiviral Drug Resistance. In addition, updated antiviral resistance figures for samples submitted to and tested by CDC are published in the “FluView” Weekly U.S. Influenza Surveillance Report.
Because of high levels of resistance to the antiviral drug, oseltamivir, found in influenza A (H1N1) viruses this season, CDC has issued Interim Antiviral Guidance for the 2008-09 influenza season.
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). Four FDA-approved influenza antiviral agents are approved for use in the United States to treat or prevent flu during the 2008-09 influenza season: oseltamivir, zanamivir, amantadine, and rimantadine. CDC has issued Interim Antiviral Guidance for the 2008-09 influenza season based on observed patterns of antiviral resistance among circulating types and subtypes of influenza.
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.
- Page last modified February 3, 2009
- Content Source: Coordinating Center for Infectious Diseases (CCID)
- National Center for Immunization and Respiratory Diseases (NCIRD)
