Choosing viruses for influenza vaccine
Again, the full answer is too complex to answer here. The CDC has a broad explanation (Selecting Viruses for the Seasonal Influenza Vaccine) and the WHO has a presentation summarizing the process (The WHO Vaccine Strain Selection Process: Review of the Evidence).  

The major factor for choosing a new vaccine virus is based on surveillance from the preceding season and from the opposite hemisphere's season. The surveillance data shows which virus groups are expanding or shrinking. Antigenic variation (high or low HI titers) is also a part of the decision, but every season has a number of viruses that have low HI titers to the current vaccine and the real question is whether those occasional outliers are just sporadic, or whether they're showing signs of becoming increasingly common. 

Interpreting whether particular virus families are increasing or not requires a lot of familiarity with influenza behavior, genetics, and antigenicity. You can look at some of the data used in strain selection on the WHO site, e.g. Recommended composition of influenza virus vaccines for use in the 2018- 2019 northern hemisphere influenza season.