Well, there are two groups roughly speaking of people with natural immunity. One group, those who have already had the virus, seem to be fairly well protected according to that one Israeli study. Obviously, going out and getting the virus to protect yourself from the virus is an idea straight out of Catch-22, so I assume you're talking about the other group. ;)
As for people with natural immunity but who haven't had it, that's everyone. Every person has what are called "naive B-cells," covered in random antibodies. One of them probably binds to coronavirus in some way, and that would be how your body would get started fighting the infection if you got it. However, the fact that people have some natural ability to fight it off explains why not everyone dies from it (every virus would kill you if you had no immune response, that's why HIV is deadly), but it's of little consolation to the people who found out too late that their natural immunity wasn't enough.
If there was a test to see in advance who would fight off the coronavirus without realizing they had it, I guess that would be a way to save some money and inconvenience giving them shots, but since we don't know for sure in advance who will or won't have an easy case, that's why we have to get vaccinated.
It would probably cost less to just administer a proven safe and effective vaccine anyway than it would to test that person even once.
3 shots of Pfizer ~ 60 USD < -- VS -- > 100+ USD ~ Single test to see if someone's infected, surely a test to see if someone is naturally immune at best costs this or more.
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Pfizer seems to cost about 20 USD per dose, so even with the three dose regime (2 initial shots plus a later booster) is expected to cost under 100 USD; most of the other shots cost less.
As there is no currently known technology to reliably test if someone is immune I'll grab data for the next closest similarity: a test to see if someone is presently infected. Nearly all of those tests cost in the range of 100 to 200 USD, with some rare outliers above and beneath.
As for people with natural immunity but who haven't had it, that's everyone. Every person has what are called "naive B-cells," covered in random antibodies. One of them probably binds to coronavirus in some way, and that would be how your body would get started fighting the infection if you got it. However, the fact that people have some natural ability to fight it off explains why not everyone dies from it (every virus would kill you if you had no immune response, that's why HIV is deadly), but it's of little consolation to the people who found out too late that their natural immunity wasn't enough.
If there was a test to see in advance who would fight off the coronavirus without realizing they had it, I guess that would be a way to save some money and inconvenience giving them shots, but since we don't know for sure in advance who will or won't have an easy case, that's why we have to get vaccinated.