Due to a process called ‘antigenic drift’, influenza viruses are constantly changing as they circulate within a population (read more at: https://www.cdc.gov/flu/about/viruses/change.htm ). The standard inactivated influenza vaccine includes multiple different subtypes of virus (from H1N1, H3N2, and influenza B viruses) and each component can change in this manner. And yes, due to manufacturing timelines, the decision of which specific virus to include in the vaccine takes place months before you actually receive the vaccine in a clinic (see Figure 1 in this publication for an example of the timeline: https://onlinelibrary.wiley.com/doi/full/10.1111/irv.12383 ). There are intense efforts underway trying to move towards ‘universal’ influenza vaccines (in other words, targeting parts of the influenza virus in the vaccine that are not subjected to antigenic drift), though this approach represents a challenging undertaking for several reasons (summarized in https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6028071/ ).