The flu (and hence the vaccine) changes and is new every year. So even if there was any vaccine left, it would be useless against this year's flu.If this is true then how could
any pre-prepared vaccine be effective? What you're saying doesn't make sense.
Aren't there a limited number of strains with very few variations?
InfluenzaEtiology
Influenza viruses are orthomyxoviruses of 3 antigenic types (A, B, and C). Epidemic disease is caused by influenza virus types A and B. Influenza A viruses are subclassified by 2 surface antigens. Major changes in the predominant strain in either of these antigens, are called antigenic shifts; minor variations within the same subtypes are called antigenic drifts. Antigenic shift has occurred only with influenza A, usually at irregular intervals of 10 or more years. Antigenic drift occurs almost annually in influenza A and B viruses.
They don't make the vaccine until after they've identified what strain they think is coming each year. So even if you get a flu shot, you may be doomed if it's the wrong strain.
You could always take last year's shot, but that would be akin to taking cough medicine for arthritis...???
Scientists make a different vaccine every year because the strains of flu viruses change from year to year. Nine to 10 months before the flu season begins, they prepare a new vaccine made from inactivated (killed) flu viruses. Because the viruses are killed, they cannot cause infections. The vaccine preparation is based on the strains of the flu viruses that are in circulation at the time. It includes those A and B viruses (see section below on types of flu viruses) expected to circulate the following winter.
Sometimes, an unpredicted new strain may appear after the vaccine has been made and distributed to health care providers and clinics. Because of this, even if you do get the flu vaccine, you still may get infected. If you do get infected, however, the disease usually is milder because the vaccine still will give you some protection.
From this link