A lot of people have been talking about it, so I thought I might do a bit of a thread on plausible reasons for the decline in COVID-19 cases in places where behaviour hasn't changed much recently 1/npic.twitter.com/QXNwGErMcN
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8/n This doesn't entirely discount the lower herd immunity threshold, but it does make it somewhat less likely What else could be happening here?
9/n I think one very plausible explanation is simply a combination of behavioural changes and some herd effects. This sounds complex, but it's actually pretty simple
10/n We tend to think of herd immunity as a fixed concept. Reach a threshold - 30%, 40%, 50% etc - and the disease stops spreading and eventually goes away This is actually only one aspect of herd immunity as an idea
11/n Herd immunity is ultimately based on a simple fact - if some proportion of people in a population are immune to a disease, the disease will spread less
12/n The threshold may be when the disease stops spreading - when the reproduction number (Rt) goes below one - but even at lower levels of immunity there will be some impact on the spread of the disease This is what I mean by herd 𝘦𝘧𝘧𝘦𝘤𝘵𝘴
13/n What does this have to do with COVID-19? Well, say we're talking about the US. Depending on which estimate you take, somewhere between 20-30% of the entire country has been infected with and recovered from COVID-19pic.twitter.com/2ttjhfIH1A
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
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
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
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
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
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
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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