Why? The version of the virus that’s circulating around the globe has undergone significant genetic changes that make it easier to infect people, and therefore to spread between them. It means “our antibodies won’t see it as well,” says Danuta Skowronski, a physician-epidemiologist and flu researcher at the British Columbia Centre for Disease Control, who first flagged the mutations in the Journal of the Association of Medical Microbiology and Infectious Disease Canada in October. What’s worse, these genetic changes happened too late to account for in flu shot formulations.