Ro and Herd Immunity are Intimately Connected
I have only just understood the intimate connection between R0 and herd immunity. I offer this explanation to others who may have shared my ignorance.
If R0 is 10, every infected person typically passes enough virus on to 10 other contacts to make them ill – unless of course they are already vaccinated or otherwise immune
But if over 90% of the population is already immune, every infected person will still pass the virus on to 10 others, but less than 10% – that is less than 1 of them – will now also become ill, and the illness will die out. The perceived R, in other words, has become less than 1.
Measles is highly infectious. Its R0 is around 15 which means that the illness will spread through a population if more than 1/15th of the population lacks immunity. This is why medics want to see around 93-94% of the population vaccinated.
COVID-19 R0 appears to be around 3. If so the illness will spread quickly if more than 1/3rd of the population lacks immunity. But herd immunity might be achieved at around two-thirds (67%) of the population.