2009-10 Influenza (Flu) Season
A season's severity is determined by comparing these measures with previous
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
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.
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
Can the seasonal flu vaccine provide protection against other seasonal flu
viruses even if the vaccine is not a "good" match?