The flu kills over 250,000 people every year. Flu viruses change constantly, so they can evade our immune systems, the immune systems of other host species, and the vaccines we throw at them. Each seasonal vaccine can, at best, protect only against the current circulating strain of virus—but not emerging variants. (Just so we’re clear on this, YOU SHOULD STILL GET VACCINATED. Reread the first sentence.) And we currently have no way of knowing which strain might become a pandemic, or when or where such a strain might arise.
Making a universal vaccine, or at least one that could counter more than one subtype of the virus, is a priority. Efforts thus far have failed, because most of the proteins that are conserved between the different influenza subtypes are inside the virus rather than on its surface, which typically makes them tough for antibodies to access. But researchers have recently found a way to render one vaccine protective against a number of different subtypes.
Influenza variation graphic from Russell Kightley Media