The cumulative proportion of individuals who get infected during the course of a disease outbreak can exceed the HIT. This is because the HIT does not represent the point at which the disease stops spreading, but rather the point at which each infected person infects fewer than one additional person on average. When the HIT is reached, the number of additional infections begins to taper off, but it does not immediately drop to zero. The difference between the cumulative proportion of infected individuals and the theoretical HIT is known as the overshoot
If, at the time we reach the Herd Immunity Threshold, many people are infected, an R number of eg 0.5 will cause a large number of others to be infected.
Wikipedia:
If, at the time we reach the Herd Immunity Threshold, many people are infected, an R number of eg 0.5 will cause a large number of others to be infected.