Specifically we guess based on the other hemisphere since flu is mostly a winter illness -- People report "flu" year round, but the deadly Influenza virus is most "flu" in winter, whereas in summer most "flu" was just a bad cold, in medicine the term is ILI (Influenza Like Illness). This gives us several months between the worst of their Influenza cases on the far side of the world and us needing to inject our vulnerable populations with a vaccine.
This means the relatively mild flu load in Australia and especially New Zealand actually hurt this year's Northern Hemisphere guess - they were isolating because of COVID-19 and so they had far fewer than usual cases on which to base guesses.
This means the relatively mild flu load in Australia and especially New Zealand actually hurt this year's Northern Hemisphere guess - they were isolating because of COVID-19 and so they had far fewer than usual cases on which to base guesses.