Medical sciences is not my subject but my personal feeling is that:

* A strengthen immune system can better respond to viruses than a weak immune system.
* A balanced diet in combination with exercise and adequate sun-light exposure strengthens the human immune system.

Is any of the above assumptions questionable?

If not, shouldn't the WHO provide any such guidelines (aside the virus-spread elimination guidelines) to better shield people (mainly elder ones) against COVID-19 infection or negative-effects of it?

I have not heard of any such guidelines (at least in Europe), but if there are, can you please provide the corresponding links?