When was the last you had long lasting immunity to a cold virus ? There have already been numerous cases of people having covid 19 more than once. Immunity after infection seems to last 4-8 months.
If you have strain X of a cold, you are immune to it, and all of its mutations for years. The reason you can get two or more colds a year is that there are so many wild strains of the common cold that are already largely unrelated to one another. That's not the case with the Coronavirus family that causes COVID19.
