Smile - all they needed ot do was do a survey in the age group and ask who got the big Hong Kong or Asian flu in 1957-58, or 1967-68. Then ask those who said yes, how often they ever got the flu afterward.
When you survive 3-6 weeks of fevers, dry heaves, etc, its seems you get a boat load of anitbodies and antibody memory.
My bet is that those who survived the 1917-1918 flu had the same immunities.
However... I still get the flu shots because when you get millions upon millions of ducks. geese, chickens and pigs together in live markets leading up to Lunar New Year (Jan/Feb) in China, Vietnam, Cambodia, and Korea - you have a live lab to create an entirely new version of flu.
And now that there is a truly global poultry market, new versions of flu will no longer be limited to Asian starts. That is what was most novel about H1N1, it started in Northern H