[T-A]

The threshold is 1 - 1 / R_0 [1]. Using the commonly quoted R_0 ~ 2.5, that's 60%. The highest estimate I've seen is R_0 ~ 5.7 [2], in which case more than 80% of the population would need to be immune.

[1] https://en.wikipedia.org/wiki/Herd_immunity#Mechanics

[2] https://wwwnc.cdc.gov/eid/article/26/7/20-0282_article