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I've been coming across quite a few papers on cross-protection conferred perhaps by previous exposure to similiar strains, so although there is not total immunity there is a less serious reaction.
we were always taught at College to look for, and concentrate more on those that survived epidemics such as this, when looking for helpful factors or indeed better treatment protocols.
A lot of this will hopefully come out more, in years to come, but already there are many instances of certain groups not succumbing through out the World; the danger is assuming that it is due to this or that 'factor', to the detriment of other possible causative factors. thus for example an area in Japan with very low rates has been thought to perhaps have protection because of their high Vit K2 intake through eating Natto -- fermented soya, but of course it could equally be a less population dense population, benefiting from the practice of 'bowing' rather than western close hand -shaking . Many other factors could also be playing a part.
this article today is a very good discussion on some of this.

"One thing seems clear: there are many reasons why one population is more protected than another. Theoretical epidemiologist Sunetra Gupta of the University of Oxford thinks that a key one is immunity that was built up prior to this pandemic. “It’s been my hunch for a very long time that there is a lot of cross-protection from severe disease and death conferred by other circulating, related bugs,” she says. Though that cross-protection may not protect a person from infection in the first place, it could ensure they only experience relatively mild symptoms."

https://www.theguardian.com/world/2020/jun/07/immunological-dark-matter-does-it-exist-coronavirus-population-immunity

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another great benefit of looking for the normal response, can be seen in a study which "documents a robust antiviral immune response to SARS-CoV-2 in a group of 20 adults who had recovered from COVID-19. The findings show that the body’s immune system is able to recognize SARS-CoV-2 in many ways, dispelling fears that the virus may elude ongoing efforts to create an effective vaccine."

"And, although these results don’t preclude that the immune response to SARS-CoV-2 may be detrimental, they provide an important baseline against which individuals’ immune responses can be compared; or, as Sette likes to put it, “if you can get a picture of something, you can discuss whether you like it or not but if there’s no picture there’s nothing to discuss.”

Information about immunity to SARS-CoV-2, both in the context of COVID-19 disease pathogenesis and in the context of how to develop a good vaccine, remains limited. But developing a vaccine and predicting how the coronavirus pandemic will unfold until such a vaccine is available are both contingent upon the understanding of whether the immune system can mount a substantial and lasting response to SARS-CoV-2 and whether exposure to other, common, circulating coronaviruses provides any kind of protective immunity.

A collaboration between the labs of Alessandro Sette, Dr. Biol. Sci., professor in the Center for Infectious Disease and Vaccine Research, and Shane Crotty, PhD, professor at La Jolla Institute for Immunology, is starting to fill in the massive knowledge gap and is providing the first cellular immunology data.Their work, published in Cell in a paper titled, “Targets of T cell responses to SARS-CoV-2 coronavirus in humans with COVID-19 disease and unexposed individuals,” studied T cell and antibody immune responses in average COVID-19 cases.

“We specifically chose to study people who had a normal disease course and didn’t require hospitalization to provide a solid benchmark for what a normal immune response looks like, since the virus can do some very unusual things in some people,”

“We have a solid starting foundation to now ask whether there’s a difference in the type of immune response in people who have severe outcomes and require hospitalization versus people who can recover at home or are even asymptomatic,” added Sette. “But not only that, we now have an important tool to determine whether the immune response in people who have received an experimental vaccine resembles what you would expect to see in a protective immune response to COVID-19, as opposed to an insufficient or detrimental response.”

https://www.genengnews.com/news/good-news-for-covid-19-vaccine-immune-system-shows-robust-response-to-sars-cov-2/