[Principled]

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.

[steve86]

Yes, and the FluMist has an expiration date at the end of the season. So, if any pharmacy had held onto it instead of returning it to the manufacturer it would be illegal to sell or administer.

[philman_36]

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?
Influenza
Etiology
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.