2009-10 Influenza (Flu) Season
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?
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.
Will the seasonal vaccine protect against the new H1N1 virus?
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.
What is CDC doing to monitor seasonal flu vaccine effectiveness for the