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(THE CONVERSATION) The current 2017-18 flu season is a bad one. Hospitalization rates are now higher than in recent years at the same point, and infection rates are still rising. The best line of defense is the seasonal influenza vaccine. But H3N2 viruses, like the one that’s infecting many people this year, are particularly hard to defend against, and this year’s shot isn’t very protective against H3N2.

Producing an effective annual flu shot relies on accurately predicting which f...