Having been working out R numbers from the government data, I was wondering about how they came up with figures for herd immunity. This is when so many people have had a virus and have antibodies that stop them getting infected again that anyone with the virus will struggle to find anyone to infect. I'd heard originally that the models for Covid 19 suggested about 60% but saw an article in The Times newspaper at the end of last month that suggested that under current conditions that figure could be as low as 15%. I was struggling to work out my own theory but I found this wonderful explanation:
https://www.quantamagazine.org/flu-vaccines-and-the-math-of-herd-immunity-20180205/ The article gives an excellent account for how to calculate the percentage for herd immunity. The formula = 1 - 1/Ro. I plotted the graph
I was confused because the R value is below 1 in the UK at the moment but the graph won't go below 1. Then it became clear that there is more than one R number. Ro is the number for an unhindered virus with no social distancing or other controls. I believe that the modellers in March were using a value of Ro for Covid 19 of 2.4, hence the 60% figure. That outs it much less contaguious than measles, which is quoted as Ro = 12.
