The flu strains are guessed at in beginning of season to determine the makeup of the vaccine. And there is some evidence that even if a different strain emerges, that maybe symptoms are less with the flu shot.
I few days ago I heard there are two strains of covid-19. I don’t know how different. But SARS in 2003 is another coronavirus that did die out. Hence the name covid-19 for this season of coronavirus. Hopefully this is like SARS and not covid-20, covid-21 like the flu.