Not a large study. But they found that people that got covid early on might not have immunity to 6 strains of the virus now.
Interestingly enough I had Covid right at the start of 2020. I didn’t know because it wasn’t serious. I was just sick for a few days.
I’m March of 2020 I had antibody tests and I was positive for all3 antibodies.
In September of 2020 I had only the long term IGG antibody.
In April 2021 I had no antibodies. I was tested before my vaccine.
So I figure in my case I had less than 2 years of protection following a mild covid infection.