From what I recall you'd need about 50% immunity for herd immunity to be reached at a minimum. Assuming 50% 3fficiency (worst case), you would need at least 13% already immune, which seems possible. But imo the efficiency might just be higher like parent said, or it might not be there yet, but deaths slow down because of the vaccine.

https://www.nature.com/articles/s41577-020-00451-5