I have heard that once you are affected by covid 19 and survive it (remain alive) then your immunity system will not let you affect by the virus again ?
How true is that ?
It's completely true. It's basic biology. You get infected by a pathogen, your body builds up immunity through antibody production which recognizes virus particles and destroys them. The only case of someone getting "reinfected" by COVID-19 is if they had the disease once and get infected by a mutated form. Your body will repeat the cycle again producing antibodies against the mutated form.
How do you know that the corona virus will not mutate? So far it's still not clear if corona will come back in the next years in a slighty different form. Just like the flu is coming back every year. If you had it in one year or even got the vaccine against it, you can still get it in the next year. I think we all should remain cautious, even if you already had corona. Unfortunately it will still take a long for us to full understand corona.