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I recently learned from an answer at health.SE* that influenza B tends to occur later in the season compared to influenza A. According to the graph in that answer, during this year’s flu season the peak for influenza B was mid-March, compared to late December for influenza A. I’m wondering why this is. What are the virological factors that account for this difference?

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Regrettably, I don't believe we know - we don't actually know why influenza is a seasonal disease in the first place. There are a number of possible theories, discussed in this review, but none of them are compelling enough to be a definitive causal explanation

In terms of the difference between Influenza A and B, B mutates at a much lower rate, which may explain the slower rate of dramatic antigenic shifts, but that isn't enough to disrupt the seasonal cycle by a few months at most.

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