> [greater activity within] our immune systems long-term can't be healthy

Not trying to be flip, but why? "Natural" isn't always better, and as the obesity epidemic has shown, our evolutionary past hasn't done a perfect job of preparing us for our current environment.

You might be right, but I'm skeptical that there is any non-extreme limit to something as simple and mechanical as our innate immune system.

It could also be useful in low doses to supplement, for example, a seasonal vaccine in a year where they are especially unsure about prevalent strains, or where their predictions were already proved wrong early in the flu season

Cant we say this applies to the flu vaccine? This almost validates why I skip it every year.

I get sick after getting the flu vaccine and feel pretty bad for 1-3 days... then I get the flu anyway because they picked the wrong ones.

you would think so! as a "vaccine skeptic", i think this kind of research is important and patients should be able to decide w/ their doctor which to pursue based on their individual condition. perhaps this tradeoff will be worth it in higher risk individuals.