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. Models might have a bad name after the last year, but ours has chimed with the official ONS estimates of the virus week after week. It can also be used to illustrate how England could relax its restrictions as more people are vaccinated.
As with all models, everything depends on the assumptions. So here goes. As three-quarters of a million jabs had been given in England by 27 December and AstraZeneca can supply 2 million doses per week, let’s suppose full-scale inoculation began on 28 December at the rate of 6.75 million injections per month in England, until 95 per cent of the population has been covered, prioritising those over 65 to start with. Let’s go with the Oxford figure; a vaccine that’s 70 per cent effective 22 days after the first dose.
Let’s also assume ministers succeed in keeping the R-rate at 0.9. These assumptions, when plugged into our model, show that infections, which went up over the Christmas period, then start their long decline — fizzling out by the end of the summer as they are driven down by the vaccinations.
The above graph shows a virus under control.