Has South Wales reached herd immunity?

Few topics during the Covid pandemic have caused more controversy than the Herd Immunity Threshold, the level of immunity at which the virus can no longer spread through a population even once social distancing is relaxed. Confident past predictions that Sweden or India had reached this have been swept away by sizeable second waves, and certainly we cannot tell from graphs of falling cases alone that herd immunity has been achieved.

However, considering the latest data, I believe it is worth asking whether parts of the UK have passed the Herd Immunity Threshold. The answer may be ‘not quite yet’, but it can be useful to think why. Essentially, herd immunity should be possible given enough infections and vaccinations, conditions that may apply in parts of South Wales, for example. But are numbers there sufficiently large, and how would we tell?

In conventional epidemic models, the Herd Immunity Threshold can be found using the famous R number.