My totally inept understanding, re: Covid vs flu
Are you familiar with languages? Think of Covid as American English and the Flu as a Romance language.
You go different places in the US, and the English is a wee bit different. Try being from the Midwest and trying to understand your Appalachian-raised music professor. There's some confusion, but they got 95% of what I said. Covid is the same way. The Delta variant has enough of the same core of what was used for the standard vaccines. If you had Covid or the vaccine before, your body can understand most of the Delta and hold its own.
Now on to the Romance languages, languages that stem from a mostly Latin and western European melding. I can read Latin and read and speak some Italian. Despite Italian coming from Latin, the structure is very different and you may be able to pick out a word here or there. Then there's French, which is Italian plus Spanish put in a blender, poured out, then every third letter omitted and you have to speak it while having a mouth full of rocks. Also a Romance language, but way the hell out in left field from Italian and Latin.
Flu strains are more like Italian and French than Midwestern English and Appalachian English. You may get the French shot, then the Italian flu shows up. Or vice versa. That's how the strains work and why you need a different vaccine depending on what seems to be coming down the pike.