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2009-10 Influenza (Flu) Season

Questions & Answers about the 2009–2010 Flu Season

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.

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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.

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 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.

How effective is the seasonal flu vaccine?

TThe 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?"

Will this year's seasonal flu vaccine be a good match for circulating viruses?

WebMD Public Information from the CDC

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