An update on the previous post on the UCL model and herd immunity levels. I just thought of adding a couple of further thoughts on this topic.
Firstly, there appears to be growing evidence of a potential for new strains (Brazil, South African, Indian) to overcome the immunity created by the previous infections.
The rapid growth of cases in Brazil and now in India suggests that either our estimates of Herd Immunity Threshold (HIT) are too low, or people who have SARS-CoV-2 antibodies can still be infected.
Secondly, there is an interesting interplay of heterogeneity and HIT. Heterogeneity, be it in the infectivity or susceptibility, is normally thought to lower the HIT. This basically is caused by the infection “burning” through the most susceptible parts of the population first.
As a result, we end up with a population where those who are not yet immune are protected by their lower susceptibility. This results in a lower value of HIT than predicted by the homogenous theory.
However, there is another trend that counteracts this process, and the one which seems more dominant at this moment. We now have a very uneven distribution of immunity due to vaccination.
Those who are potentially most likely to become infected (those in the BAME population, in deprived areas, or those who are vaccine-hesitant or anti-vaccine) also have the lowest vaccination coverage.
Thus, even if on average we can reach the HIT, these pockets of low-immunity, high-susceptibility/high-infectiousness, will keep the infection going and possibly creating the conditions where new strains can arise.
Even more arguments for the #ZeroCovid strategy… Let’s just keep going for a while longer, paying particular attention to those who are most at risk, to reduce the overall levels of infection until we can bring the vaccination higher.