Thread by @Marc_Veld

Population immunity. Yes, that again. A very difficult concept for many. So much so that some claim it does not exist or, at least, doesn't for SARS-COV-2.
But it does, of course. It is misunderstood, bc it is not how many understand it should be working.
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Again: immunity reduces the risk of disease as a consequence of a particular infection.
Risk of infection (and disease) depends on the host (antibody waning) and the pathogen (mutations, variants location and speed).

(For disease: pathogenicity, speed, load, genetics)
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This causes confusion with many thinking that immunity prevents infection: it can, but during a limited time.
Yet, this is often used to explain group immunity. It is not incorrect, however, think about the pathogen side!

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Many can think of pathogens like measlesvirus, and think they have not been infected bc they were vaccinated or had the infection. But you do, but clear the virus before symptoms or transmission. You protect others.


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For a fast respiratory virus ithere is still group immunity. Yes, it reduces disease and also infections. The infection reoccures more frequently and you notice them more. The pathogen is very fast and can transmit.

?
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This means that protection of those around us is less. But it still matters!

Our rapid response may not stop infection or new virus generation. But, it does reduce transmission. It does this by killing infected cells: less virus production. And by coating the virus
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with antibodies. Many will be neutralising and thereby reduce transmission. Other will make the particle more immune visible and ensure rapid clearance. The total means: less risk of transmission and less infectious virus when transmission does occur.
This matters a lot!
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It matters a lot when you think about exponential growth. Even reducing that with 10% makes a big impact.

But there is more. Synchronised immunity does mean many of us are not or with more difficulty susceptible to infection for several months. The classic group immunity.
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Yes, true, that will wane. But it pushes the virus into a wave pattern, and with immunity strong, the infection rate will be more determined by human behaviour: the virus strikes when antibodies have waned sufficiently and we are closer together with less ventilation.
9/10

This is seasonality. Ensuring we do not have a high risk of infection the whole year round.
This is how it works for all viruses, why we have higher risk of infection at some points of the year over others. SARS-COV-2 is no exception. But, it does take a bit of time
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