Israeli Maccabi retrospective cohort during the Delta period: previously infected, unvaccinated people had substantially lower reinfection risk than twice-vaccinated, never-infected people; symptomatic disease and hospitalization were rare in both groups (8 hospitalizations, all in the vaccinated group; zero deaths).
SARS-CoV-2-naive vaccinees had a 13.06-fold (95% CI, 8.08 to 21.11) increased risk for breakthrough infection with the Delta variant compared to those previously infected
Clin Infect Dis 2022;75(1):e545-e551 (PubMed abstract)
Israeli Maccabi retrospective cohort during the Delta period: previously infected, unvaccinated people had substantially lower reinfection risk than twice-vaccinated, never-infected people; symptomatic disease and hospitalization were rare in both groups (8 hospitalizations, all in the vaccinated group; zero deaths). PMID 35380632. Observational Delta-era design; the 13-fold figure is for documented infection, not severe disease, and the authors note confounding and behavior differences as limitations.