I'm not aware of any evidence of immunity after recovery waning over time. Omicron seems to infect more people with a previous infection than delta did, but protection from hospitalization is still strong. I'm interested to see otherwise if you have links.
The CDC looked at how people fared with delta with a previous diagnosis by March 2021, so presumably anywhere from when pcr and antigen tests started up until March. There is no breakdown of previous variants in this study, its just if you had a previous infection, you were protected.
I know, this doesn't sound like anything you've heard yet. They still manage to slip in a recommendation that "all eligible persons should be up to date with COVID-19 vaccination", but the data doesn't support that.
> Four cohorts of persons aged ≥18 years were assembled via linkages of records from electronic laboratory reporting databases and state-specific immunization information systems.† Persons were classified based on whether they had had a laboratory-confirmed SARS-CoV-2 infection by March 1, 2021 (i.e., previous COVID-19 diagnosis)§; had received at least the primary COVID-19 vaccination series¶ by May 16, 2021; had a previous COVID-19 diagnosis and were fully vaccinated*; or had neither received a previous COVID-19 diagnosis by March 1 nor received a first COVID-19 vaccine dose by the end of the analysis period.
> By the week beginning October 3, compared with COVID-19 cases rates among unvaccinated persons without a previous COVID-19 diagnosis, case rates among vaccinated persons without a previous COVID-19 diagnosis were 6.2-fold (California) and 4.5-fold (New York) lower; rates were substantially lower among both groups with previous COVID-19 diagnoses, including 29.0-fold (California) and 14.7-fold lower (New York) among unvaccinated persons with a previous diagnosis, and 32.5-fold (California) and 19.8-fold lower (New York) among vaccinated persons with a previous diagnosis of COVID-19. During the same period, compared with hospitalization rates among unvaccinated persons without a previous COVID-19 diagnosis, hospitalization rates in California followed a similar pattern. These results demonstrate that vaccination protects against COVID-19 and related hospitalization, and that surviving a previous infection protects against a reinfection and related hospitalization. Importantly, infection-derived protection was higher after the Delta variant became predominant, a time when vaccine-induced immunity for many persons declined because of immune evasion and immunologic waning (2,5,6).