It has to do with the flu virus' "drift" and "shift," said Fauci, who's part of the effort to fight future outbreaks. What's called antigenic drift has to do with small changes in the genes of the influenza virus year to year, while an antigenic shift has to do with a major change, which can cause pandemics.

"This is so different from other important viruses like measles. The measles I got as a child is the same measles that's in the vaccine that we vaccinated my children with. So you don't have to worry about that change," Fauci said Tuesday on "CBS This Morning." "So what we're fighting against is trying to get a [flu] vaccine that would induce a response against that part of the virus that doesn't change from season to season or with a pandemic. We refer to that as the universal flu vaccine."

https://www.cbsnews.com/news/flu-outbreak-why-isnt-there-a-universal-vaccine/