Re: R numbers and Herd Immunity concepts
The spirit of my post is clarity about ‘the science of the numbers‘ and aims to be helpful in understanding how figures are reached from a medical perspective (it is apolitical); if it’s helpful - good , if you know it already - skim . If you want to argue the political interpretations ….. I can’t help
R0 number is well known = basic reproduction number = average number of 2nd infections from a single infected person introduced into completely susceptible population.e.g 4 each person infects 4 others etc etc. Less than 1 - insufficient to sustain transmission.
Versus
Re - average number of 2nd infected persons generated by index case over infectious period in partially immune population. This number changes dynamically over vaccination campaign.
HERD immunity- when enough people are immune to an infection then even the non immune are protected because number of latter too small for it to spread.
HERD immunity calculation -
Required % of population who need to be immune to induce herd immunity in the remaining population = 1-(1/R0)
e.g Delta R 4-5 est (using 5)
= 1 - 1/5 = 1-0.2 = 80%
another example is Measles with R of 12 which is why about 92% immunity needed.
goal of vaccine is to bring Re below 1
Just a quick overview in a v simplified way…
but….. I won’t depress you with all the complex caveats that mean you can achieve these numbers but not predict the outcome nor achieve the goal
with certainty.