Julian wrote:(*) It seems to me that it's meaningless to quote a specific figure for herd immunity in isolation. 70% of the population with immunity from vaccines? Is that 70% regardless of whether those people were vaccinated with a vaccine that offers 50% protection against the prevalent strain vs one that offers 95% protection? I would say clearly not. Any figure for what percentage of the population needs to have immunity in order to reach overall herd immunity must surely be a function of the level of immunity in that percentage of the population with immunity? And the level of immunity isn't a constant across that group of people with immunity either, people will have had different vaccines with different efficacy characteristics and others will have acquired their immunity via natural infection. And it is not only the levels of efficacy but potentially immunity from different sources will have different decay characteristics. So essentially any figure bandied about, e.g. 80%, must be in the context of an assumed immunity profile for those with immunity i.e. that figure is in fact a function of a given immunity (efficacy) profile where that immunity profile is in itself an extremely complex variable (not in the sqrt(-1) sense!). I assume it's also affected by the transmission properties of a particular virus so is also specific to a specific virus. The simple percentage numbers we see all over the place must, I think, hide an extremely complex calculation that epidemiologists are presumably doing in the background that we don't get to see.
The complexity would be in the range of different levels of protection. If it were simply that, say, 70% of the population had a vaccine that was 70% effective and everyone-else was 100% susceptible, the level of immunity would simply be 70% x 70% = 49%.
Back in March 2020, we were told that the strategy was herd immunity (through infection). This seems to have been part of the policy ever since, even after effective vaccines received emergency approval. I do not know of a reliable estimate of the number of people who have been infected but these data suggest that it could be quite high:
The estimated share of the population with SARS-CoV-2 antibodies in the week beginning 7th June 2021 ranges from 79.1% (Scotland) to 88.7% (Wales) https://www.statista.com/chart/23961/uk-share-with-covid-antibodies/. While this is among over-16s in the community, I suggest that the figures are similar for the whole of the population.
Only about 60% of the UK population had received at least one vaccine dose.
Infection + vaccine + vaccine is claimed to confer more immunity than just vaccine + vaccine, and I think it most likely that vaccine + vaccine + infection will also confer a high level of immunity. Add another jab in the Autumn* and, hopefully, any new variants will have even less of a harvest.
Julian F. G. W.
*While I would probably accept a third jab if offered, I would prefer these jabs to be offered to other countries while there is a shortage. New production facilities should come online after that and allow additional jabs (hopefully tailored to newer variants) to be offered in the new year.