To add to this, the effective reproduction number (what I think the GP is referring to?) varies not just with the prevalence of immunity, but also with the contact network of the population in question.

Currently the effective reproduction number is not 3, in most places, because we're avoiding large gatherings and, depending on the region, wearing masks and avoiding indoor gatherings.

If many more people were immune, even if we hadn't reached herd immunity, we could safely relax restrictions on gatherings without causing another wave. Concerts would still be out of the question, but dinner parties and hair salons would no longer be so concerning.