The multinational team from Stanford, France, and the University of North Carolina
Good old Ralph Baric
took blood from 54 women and 37 men of different ages,
The Lord Humungous of studies.
then studied a variety of immune system proteins and cells using complex systems to detect gene expression.
Oooh! Sciencey!
Then they gave flu vaccines to these people and checked for any changes in these parameters
The aformentioned parameters' changes in response to a flu vaccine being identical to, or at least representative of, the immune system's response to the influenza virus, presumably. Which is the million dollar question here but we'll just take it on faith, k?
Sure enough, men, as a group, had a more muted response to the vaccine.
And therefore immune response to the flu also, you'd like us to infer. Which is totally borne out by clinical results where guys in their 20s die of the flu in droves. But not.