Depends what you mean by "handled". Per the CDC [1]:
> [vaccine efficiency at preventing infection] declined from 91% before predominance of the SARS-CoV-2 Delta variant to 66% since the SARS-CoV-2 Delta variant became predominant
Per OP:
> Because the Delta variant is spreading so explosively, it has already acquired numerous additional mutations in the spike protein coding region, suggesting that the Delta variant will continue to acquire further mutations.
> A third round of booster immunization with the SARS-CoV-2 vaccine is currently under consideration. Our data suggest that repeated immunization with the wild-type spike may not be effective in controlling the newly emerging Delta variants.
[1] Effectiveness of COVID-19 Vaccines in Preventing SARS-CoV-2 Infection Among Frontline Workers Before and During B.1.617.2 (Delta) Variant Predominance — Eight U.S. Locations, December 2020–August 2021
https://www.cdc.gov/mmwr/volumes/70/wr/mm7034e4.htm
This isn't surprising. Vaccines are not expected to prevent infection, and almost none do. Rather, they are designed and tested to prevent disease.
The focus on preventing infection is understandable, but I feel it's largely counterproductive in the national discourse. I wish there were more emphasis on the vaccines' ability to prevent the disease of covid, and the evidence still shows that they are doing this job well.
Delta remains handled by current mRNA. Combine 4 seen mutations (present in Delta+) and it has complete breakthrough.