I have read this article about how difficult is to send ships to another possible habitable world. One aspect deals with the human bacterial companions which might be greatly affected by prolonged living in a very closed environment:
We are always teamed with many other living creatures. Eighty percent of the DNA in our bodies is not human DNA, and this relatively new discovery is startling, because it forces us to realize that we are not discrete individuals, but biomes, like little forests or swamps. Most of the creatures inside us have to be functioning well for the system as a whole to be healthy. This is a difficult balancing act, and does not work perfectly even on Earth; but divorced from Earth’s bacterial load, and thus never able to get infusions of new bacteria, the chances of suffering various immune problems similar to those observed in over-sterile Terran environments will rise markedly.
As reaching a habitable planet outside our solar system is estimated to decades, if not more than 100 years, I expect these effects to be less noticeable for short periods of time in space.
This article confirms that gut microbiome is changing while living in space, more related to microgravity rather than radiation. However, it is not clear if these effects might lead to such serious health issues that a very long trip in space is virtually impossible.
Question: Were there any noticeable immune system changes observed for people staying for long in space?