There is a long-standing belief that herd immunity can only be achieved once between 60 to 80 per cent of the population is immune. This notion, however, does not take into account the fact that immunity can be achieved at a lower figure if social distancing measures are factored in. The herd immunity threshold, in fact, is not fixed — it depends on the amount of social distancing we are doing, and, specifically, on what epidemiologists call the 'basic reproduction number', known as the R0-value.
A PCCF (standing for 'predictor-corrector coronavirus filter') measurement tool is under development at the University of Bristol. It has successfully projected the trajectory of the virus, making use of models to help it do so.