On January 31, 2019, an 11-year old boy in Japan went to a medical clinic with a fever. The providers there diagnosed him with influenza, a strain called H3N2, and sent him home with a new medication called baloxavir.

For a few days, he felt better, but on February 5, despite taking the medication, his fever returned. Two days later, his 3-year-old sister also came down with a fever. She, too, was diagnosed with H3N2 influenza on February 8.

An analysis of flu samples collected from her and her brother show she was sickened by a strain of H3N2 harboring a new kind of mutation -; one that Yoshihiro Kawaoka, University of Wisconsin–Madison professor of pathobiological sciences, says is resistant to baloxavir, is just as capable of making people sick as the non-mutated version, and is capable of passing from person to person.

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