Herd immunity and the reproduction rate are closely linked. R

o is the reproduction rate at the start of an outbreak when nobody has immunity. It is simply the average number of people that an infected person will pass the disease onto if nobody has immunity. Assuming people who catch the disease and survive become immune, which is not certain, as more and more people get immunity the probability of someone with the disease meeting someone without immunity decreases and so the effective reproduction rate, R

eff goes down. Once R

eff gets less than 1 the disease starts to decrease and dies out. The smaller R

eff the faster the disease dies out.

For COVID-19 the best estimate for the percentage of the population that need to have immunity to achieve an R

eff of less than 1 is about 60 %. The estimate is a bit variable because we don't know the exact value of R

o. That level is defined as herd immunity.

It is true that herd immunity

could be achieved by letting the virus spread. That might be a very sensible option if the Case Fatality Rate, CFR, is 0. If nobody is going to die why worry? In the case of COVID-19 we know that the CFR is definitely not 0. Amongst the older demographic the CFR is estimated to be above 10 %. Hence my question, how many deaths are acceptable?

This is a statement from an

ONS report:

Between 26 April and 26 July, 6.2% of people tested positive for antibodies against SARS-CoV-2 on a blood test, suggesting they had the infection in the past.

That is based on widespread sample testing across the population of England. Basically because that is based on an antibody test that detects people who have had COVID-19 at any time in the recent past, i.e. not just people who have it now, it represents the percentage of the population that have had COVID-19 since the start of the pandemic up to 26 July. Given the new infection rate since the 26 July has been relatively low compared to the peak of the pandemic there is absolutely no way that the number of people who have previously been infected could be above 60 %. That is also born out by the fact that as lockdown measures are eased the infection rate starts to increase so R

eff in the absence of mitigating measures is still above 1. If herd immunity had been achieved you could totally remove all mitigating measures and the infection rate would continue to decline.

People are tending to believe whatever information allows them to do what they want to do without really questioning if that information is accurate.

Warwick