Jacqueline Katz, the Deputy Director of the CDC’s Influenza Division and Director of the U.S.’s WHO Collaborating Center for Surveillance, Epidemiology and Control of Influenza, explained last Thursday that they’ve been tracking one mutation in particular for months now. “When I talked to you back in October, I mentioned the emergence of a B Victoria deletion variance,” she said. “Fast forward through to the beginning of this year, and we now see that these viruses have spread to several Central and South American countries. We see them throughout North America, and a number of European countries are now reporting their detection.” That variance is a sequence of two amino acids that have been deleted from the B Victoria genome. It’s a very small change, but the double deletion is different enough from the B viruses in our current vaccines that we’re unprotected from this new variety.