As flu season gets underway, global health experts are increasingly worried about a new strain of the virus that popped up in June — four months after the makeup of this year’s flu shots had been decided.

The new strain, a version of H3N2, is causing outbreaks in Canada and the U.K., where health officials are warning about the early wave that’s sending people to the hospital.

“Since it emerged, it’s rapidly spreading and predominating in some countries so far in the Northern Hemisphere,” Dr. Wenqing Zhang, head of the World Health Organization’s Global Respiratory Threats Unit, said Wednesday during a media briefing.

The version of H3N2 that’s circulated worldwide this year “acquired 7 new mutations over the summer,” Antonia Ho, a consultant in infectious diseases at Scotland’s Unive

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