"I mean, I had never heard that the flu would kill somebody like her", says Gwen Zwansiger, who lost her daughter Shannon to the virus three years ago. Within a week of coming home from school with a sore throat, Shannon's organs were destroyed by the virus. "Every year, you have to reevaluate whether the vaccine that you made for the prior year is actually now matched to the virus that you predict will be circulating in the coming year", says Dr. Anthony Fauci, head of the Infectious Disease division of the US National Institutes of Health. "That's totally unique. You don't have to worry about that with polio or with mumps or with measles or anything like that", he says. Here, he speaks about flu - but as new strains of coronavirus emerge, the problem of mutation is becoming clear.