We have previously seen that those with prior covid have, on average, some neutralizing antibodies against the delta variant at six months but not one year.
This study done in Thailand looks specifically at those who had prior severe covid (defined as requiring hospitalization). These people tend to have higher antibodies than those with milder disease.
They had blood samples from those hospitalized in March-May 2020, and April-May 2021. They also had blood samples from people vaccinated with CoronaVac, which is an inactivated virus vaccine designed in China. It's a fairly weak vaccine, so it's not surprising that the neutralization is weaker than those who had severe disease.
This is a pseudovirus neutralization assay, so take it with a grain of salt, but they are actually finding less neutralization for the delta variant than the beta variant using antibodies from those with prior infection. This is the reverse of what we are seeing for those with antibodies from vaccination with the RNA vaccines. It may be an artifact of the assay, but if it's real, it would help explain why the delta variant is able to spread so well in populations where the alpha variant is not able to spread.
The good news is that the stronger antibody response means that those with prior severe covid appear to have more remaining antibody neutralization than do those with milder disease.
Link to study : https://www.medrxiv.org/content/10.1101/2021.07.10.21260232v1.full.pdf