That explains tiny variations, but the article is talking about a shift to a new variant which requires a different immune response. The question is why did th

That explains tiny variations, but the article is talking about a shift to a new variant which requires a different immune response. The question is why did this new variant "completely replace" the old ones in the population, instead of just operating in parallel? The only thing I can think of is that the old flu variants were in decline already due to herd immunity, but the article seemed to imply there was more to it than that.