At 20:15 14/03/2020, Thomas Cox wrote:
>As best I understand, herd immunity is based on prior exposures to
>similar diseases, and it includes vaccinations and social distancing
>as some of many measures at containment.
Herd immunity refers only to immunity mediated by the immunological
system - it has nothing to do with social distancing/'measures'
etc. The immunity in an individual develops as a result of their
immunological system being exposured to the pathogen in question
itself or to something sufficiently antigenically similar to the
pathogen that the antibodies produced will also be active against the
pathogen if/when it is encountered again. The stimulus to the
generation of immunity can obviously be either 'natural' (exposure to
the actual pathogen) or result of administration of a vaccine.
>So herd immunity, I believe, usually builds over time, rather than
>being an immediate response to a new illness.
Immunity in an individual appears very rapidly after exposure to the
pathogen or vaccine. Herd immunity simply refers to a sufficeint
number of people in a population being immune to prevent an epidemic occurring.
>There is absolutely nothing to indicate that when 60% of the world
>population is infected with Covid-19 .... nobody else will become
>infected, or that the rate of infection will slow due to any other
>mechanism than running out of available hosts .
Once the herd immunity threshold is reached (within any population -
be that a country or the whole world), that means that it is unlikely
that any large-scale epidemics will occur. It does not mean that
'nobody else will be infected', but it does mean that any 'small'
outbreaks which arise will spread much more slowly than if the level
of immunity in the population were lower. That is basically just
common sense - the fewer the number of contacts of an infected person
who are susceptible to being infected, the less often will the
infected person infect someone else.
Kind Regards,
John
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