https://wwwnc.cdc.gov/eid/article/27/9/21-1042_article
"Seroconversion" is the technical term for when the humoral immune system begins producing antibodies subsequent to an infection or vaccination.
There's a paper in the journal "Emerging Infectious Diseases" last month that indicates the seroconversion rate subsequent to SARS-CoV-2 infection might be as low 64%.... meaning that up to 36% of infected people potentially never develop the antibodies necessary to combat re-infection.
I think caution is warranted against over-interpreting the results of this paper... we don't yet know enough to conclusively state that a lack of seroconversion equates to lack of protection against subsequent infection.... but the evidence thus far is definitely pointing that direction.
It also merits mention that, while I don't have the paper in front of me, it's my understanding that there is tremendous variation in the the humoral immune response to infection even among those that show evidence of seroconversion.
Short form: get vaccinated, even if you think you've previously had COVID-19.