For anyone curious,
R0 = 3 means iff 2 in 3 people (66.66%) [0] are immune would R0 drop to 1 [1].
So, when R0 = 5.7 (the value proposed by tfa for SARS-CoV-2), 4.7 in 5.7 (82.45%) would need to be immune for it to drop to 1 [1].
Why should R0 drop to 1 [1]?
It means each case leads to only 1 successful transmission of the infection. This implies constant incidence over time. If a greater proportion are immune, then incidence will decline.
Why "herd immunity" isn't enough?
...problems arise because herd immunity is not the same as biologic (immunologic) immunity; individuals protected only by indirect herd effects remain fully susceptible to infection, should they ever be exposed. This has advantages, in protecting individuals with contraindications to vaccination or those who for other reasons miss vaccination, but it also has its disadvantages. Measles and mumps outbreaks among university students, and pertussis in adults, are among examples of the consequences of accumulation of susceptible individuals who have not been protected by vaccination, and escaped infection because of a herd immunity effect earlier in their lives... This means that there is a need for immunization programs to maintain high vaccine coverage, together with surveillance and outbreak response capabilities, as numbers of susceptible individuals accumulate in older age groups.
From: https://academic.oup.com/cid/article/52/7/911/299077
[0] This assumes a 100% effective vaccine (E = 1) in ((R0 − 1)/R0)/E. If, for a given vaccine, E < (R0 - 1)/R0, it'd be impossible to eliminate the infection through that vaccine. (R0 − 1)/R0 is known as the "herd immunity threshold."
[1] What I really mean is R0 x S = 1. See: https://en.wikipedia.org/wiki/Endemic_(epidemiology)