Interesting study from Nature that helps give a glimpse at the immunological data for Delta in previously infected and vaccinated individuals.
Previously infected should get vaccinated against Delta. They have a higher risk of reinfection than with Alpha.
The Orléans cohort was a study of previously infected. The Strasbourg cohort looked at both previously infected (top middle) and previously infected vaccinated (top right).
At 6 months post symptoms (M6 POS) 76-92% of people were "neutralizers" shaded in blue with neutralizing antibody titers above 30 (top left). At 12 months post symptoms (M12 POS) there was a significant reduction in the percentage of people that were neutralizers against Beta and Delta, with only 47% neutralizers against Delta (top middle).
For previously infected that got vaccinated, all individuals were neutralizers against all variants (top right).
For people that were not previously infected who were vaccinated, partial vaccination was not very protective against Beta or Delta (middle left and bottom left). Full vaccination however was (middle middle and bottom middle).
This immunological data isn't a perfect correlate to vaccine effectiveness, but it can give some idea of protection. It helps explain why the effectiveness against infection after one dose isn't enough for Delta.
Link to study: https://www.nature.com/articles/s41586-021-03777-9
Here are the titer graphs for the strasbourg cohort
