Researchers have been trying to figure out why some people seem more resistant to certain flu strains than other people. Combing through hospital records, they think the answer is related to what strain you were first exposed to as a child.
There are different families, or strains, of the flu. Your first exposure to the flu gives you a big boost against that same strain later in life. However, your second encounter with the flu, particularly with a different strain, does not give nearly the protection against later encounters as with your first.
This information could help hospitals better decide who should be given vaccines that are limited in availability. In the 2017-18 flu season, 80,000 people died in the US alone and this could help lower death rates in the future.