[lookACamel]

"Everything being connected" is neutral. By itself that tells us nothing about whether having had the flu will have positive or negative effects in the long run. Why then are you so quick to bias yourself towards the idea of flu have some sort of protective effect down the road?

To see how "everything is connected" is, by itself, devoid of content, let's rewrite your comment:

Old people should take it but young people shouldn't forget to do so as well: flu might leaving lasting damage to the immune system We don't know very well how the body works on the higher "interconnected" level of how "X affects Y down the road over time" so allowing yourself to accumulate damage from constantly getting the flu once a year might end up causing severe unexpected issues down the road.