and there are vaccines for those annually - the most recent being "tetravalent" (with an annual combo of 4 of what are anticipated to be the predominate variants in circulation during a flu season).
The expectation is that the same type of process will evolve for this. They are aiming to get "combos" developed (these "bivalent" versions are the first going that route), and at some point in the future, the shots will probably move to biannual, and then annual.
The one unique thing about the mRNA vax development and changes for variants vs the flu ones, is that the mRNA ones can have the circulating variants ID'd up to 3-months before a fall season and get enough manufactured in time, whereas because much of the manufacturing for the flu vaccine relies on chicken eggs, that may need upwards of 6 - 7 months after identifying a strain/variant, to get enough ready for distribution. So doing it with mRNA is more "nimble" and the companies using that tech have been testing manufacturing flu shots that way vs using eggs (and vs making a whole lot of vaccine for a variant that ends up not being the one that hits the northern hemisphere despite being prevalent in the southern hemisphere during their summer).. 
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