What starts out as an influenza (Flu) A/H3N2 season may end up being a Flu B/Yamagata season. Or vice versa. Or with different players altogether. Flu is tricksy.
We see this often. It is normal. It is a major reason we have a Flu vaccine that aims to protect us from multiple Flu virus subtypes.
As a Flu season rolls on in a given region, Flu types and subtypes can exchange dominance. We can see this just happened during the first 2 weeks of 2018 in China. The Flu B (Yamagata lineage viruses) dominated late last year in China, but these have now been surpassed by Flu A strains – in particular, those of the A/H1N1 subtype.
More and more of the population become infected (or have been successfully vaccinated), and then developed immunity to infection by one Flu strain and so, as time goes on, there are fewer susceptible people in the community able to be struck down by the same strain. Another strain takes over. And so it goes.