"...or whether the mild cases are just a consequence of the virus infecting people who are partially immune" There are not a lot of people out there still who

"...or whether the mild cases are just a consequence of the virus infecting people who are partially immune" There are not a lot of people out there still who are not partially immune at this point, due to vaccine, recovery or both. It varies by region of course, but it's certainly true in most parts of the US.