They do strain selection twice a year (once for northern, once for southern hemisphere). The years where they get it wrong are usually the years where the strai

They do strain selection twice a year (once for northern, once for southern hemisphere). The years where they get it wrong are usually the years where the strain jumps between different clusters. It's really interesting stuff. Take a look at https://www.whocc.infectiousdisease.cam.ac.uk/antigenic-cartography https://www.whocc.infectiousdisease.cam.ac.uk/antigenic-cart... for a bit of an overview on that stuff.