The exact composition of seasonal influenza vaccines is decided semiannually (for vaccines prepared for use during flu season in the northern hemisphere, and then again for use in the southern hemisphere, because their flu seasons happen at different times). Once that decision is made (in collaboration by the WHO and various national authorities), it is very difficult to ‘update’ the vaccine composition due to tight manufacturing deadlines associated with producing sufficient quantities of inactivated vaccine for widespread use. This is why influenza virus surveillance year-round, world-wide, is so important: it gives the subject matter experts who make decisions about choosing specific candidate vaccine viruses the most information possible to make evidence-based decisions regarding vaccine composition that will be a close a match to circulating viruses during flu seasons as possible.