FROM ANOTHER FORUM:
.......
A friend of mine runs a large university department, much of which deals with this type if thing. He specialises in tsetse fly and transmission of sleeping sickness between species.....
Anyhow, heres what he's posted on our watts app group. A simple explanation of 'herd immunity' that I can understand.
From Matthew -
Much talk now about "herd immunity" - what is it and how much is it? In a nutshell, everyone in a population is safe from infection before all of them are immune. The reason is that at a certain level of immunity (ie, a proportion of people are immune, from having had the disease or having been vaccinated), the point comes when - on average - one infected person does not manage to contact and pass the infection on to one other person. Most of their contacts are already immune. The occasional contact is still susceptible, and the odd transmission happens, but not enough to keep the disease going. Transmission grinds to a halt, even though some people have still not had the disease. This is herd immunity.
It is one of the reasons boys are vaccinated against rubella: by vaccinating boys, boys are less likely to transmit to girls (effect of immunity), and by vaccinating boys, girls are less likely to transmit to girls (effect of herd immunity).
So, key question is how much immunity is needed before we get herd immunity? It varies per disease, depending on how transmissible it is. For a highly transmissible disease, like measles, on average one person infects ~20, and herd immunity kicks in at 95% immune [hence, target coverage for MMR vaccine is 95%]. For flu, on average one person infects another two; this means herd immunity kicks in at about 50% immune [hence, target coverage for flu vaccine is nearer to 50%; so it is only make available to the more elderly].
So what about covid19? Estimates are that one person may infect as many as 3-6; meaning herd immunity will kick in at 67 - 83% immune. Hence, in the absence of vaccine (1 year away) there is nothing to stop the spread of covid19 until 67-83% of us have already had it; and at that point herd immunity kicks in and transmission will decline/disappear as if by magic.
This is where the "80%" of the population statistic comes from.