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(Warning: very layperson understanding below)

There are many variants of the flu. Most of them are well-enough understood to vaccinate against, and most of those have an already-tested-and-approved vaccine. The yearly flu vaccine is a combination of those vaccines for the strains the public health experts think are going to be an issue that season.

Sometimes they get it right. Sometimes they don't, and we have a bad flu season (see also: this year).