The studies:
https://www.medrxiv.org/content/10.1101/2021.08.24.21262415v...
https://academic.oup.com/aje/article/191/8/1420/6556183
https://pmc.ncbi.nlm.nih.gov/articles/PMC8627252/
There are many more.
Several 2021–2022 studies, especially Delta-focused, suggested natural immunity provided robust or superior protection against reinfection compared to two-dose vaccination alone.