It means the population as a whole would have herd immunity (on average, every infected person would infect significantly less than 1 new person).
Result=> epidemic dies out.
Result=> epidemic dies out.
Vaccines get you the individual protection (and eventually herd immunity) without all the death.
The “standard example” for natural herd immunity eradicating a disease is the black plague, but most experts now believe that herd immunity isn’t actually responsible for its disappearance from Europe.