3/n The explanation proposed by some has been that these places have reached "herd immunity", essentially a threshold where enough people have been infected and recovered such that the disease can no longer spread
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14/n In addition, we know from various data sources (this is Google mobility data) that while behaviour might not have changed substantially since late last year, it still isn't *normal* from a 2019 point of viewpic.twitter.com/pOaesOGGZR
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15/n You also have to consider that even mobility doesn't paint a fulsome picture of how our behaviour has changed while living through a global pandemic. There are many minor differences in our lives that may at a population level have an impact on disease spread
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16/n What this means is that it's not unlikely that rather than the traditional R0 of COVID-19 of ~2.5-3, the US is really looking at a number a bit lower than that even before you take immune people into account
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17/n And given that somewhere around 30% of the entire country is immune to the disease - vaccinated+recovered - we are starting to see herd effects come into play This is speculative, but I think a reasonable argument to makepic.twitter.com/ruII2nJX21
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18/n The big caveat to this seemingly good news is that the declining numbers require people in the US to maintain various cautious behaviours until the vaccinated population is much bigger
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19/n If this speculation is correct, and everyone entirely dropped their guard against COVID-19, even in places that were very hard-hit you'd expect to see another resurgence of the disease As we did in Manaus
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20/n TL:DR I think a reasonable explanation for declining COVID-19 cases in a number of places is a combination of long-lasting behavioural changes alongside enormous numbers of infections. Hopefully vaccines can bridge the remaining gap in the near future
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End of conversation
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