In light of the most credible reinfection case to date (👇) there has been discussion of what this means for herd immunity from infection or vaccines.
Quick thread with a simplified explanation of the basic concept of population immunity.🧵 1/8
Quick thread with a simplified explanation of the basic concept of population immunity.🧵 1/8
What does it take for an epidemic to die out, so that a people are no longer being infected in significant numbers?
When first confronting this question, it might seem like the answer is that nearly everyone has to be protected from infection, so that any infected person is...2/
When first confronting this question, it might seem like the answer is that nearly everyone has to be protected from infection, so that any infected person is...2/
unlikely to infect anybody else.
However, it is not required that each infected person infects 0 new people for an epidemic to be unsustainable, we need only that each infected person infect less than 1 person on average, so that new infections are outnumbered by recoveries. 3/8
However, it is not required that each infected person infects 0 new people for an epidemic to be unsustainable, we need only that each infected person infect less than 1 person on average, so that new infections are outnumbered by recoveries. 3/8
The average number of people infected by each infected person is affected, among other things, by the fraction of the population with immunity to infection, and the effectiveness of that immunity at preventing reinfection. 4/8
But since population immunity requires getting the avg number of new infections per case under 1 rather than all the way to 0, population immunity can be achieved without 100% of the population having immunity, and without the immunity that people do have being 100% effective.
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Note that everything in this simplified discussion holds regardless of whether immunity is granted by vaccines or by infection.
Population immunity is an emergent phenomenon, where a population is protected even without 100% of individuals being 100% protected. 6/8
Population immunity is an emergent phenomenon, where a population is protected even without 100% of individuals being 100% protected. 6/8
It is the basis not only of infection-granted population immunity but also vaccine-granted population immunity.
Note that two doses of MMR are roughly 97% effective at protecting against measles infection, not 100%, but population immunity can still work!
7/8
Note that two doses of MMR are roughly 97% effective at protecting against measles infection, not 100%, but population immunity can still work!
7/8
So if you hear that it is "possible" for reinfection to occur after COVID infection, or that a vaccine candidate is less than 100% effective at preventing COVID infection, neither fact means that herd immunity cannot control COVID, whether granted by infection or vaccine. 8/8
