Thread by @GidMK

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/n

2/n The basic background is that there are some places across the world where there hasn't been a reportedly huge behavioural change since Nov/Dec last year where cases are dropping, sometimes quite quickly

So what's causing this?

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

4/n Is this likely?

Well, we know that these locations almost certainly haven't reached a traditional herd immunity threshold. Serology and other data points to most places in the US and Europe as being far below the 60-70% infected required

5/n It's also probably not vaccines. While vaccine efforts are ongoing in all of these places, few countries have managed to vaccinate more than a small % of their citizens (with some very big outliers)

6/n Some have proposed lower herd immunity thresholds, that only require 30-40% of the population to be infected. Now, it is ~possible~ that these have been reached and this is causing the decline

However I'm not sure it's likely

7/n We know from places like Manaus that not only can a large proportion of people in the population get infected, but that you can see a massive epidemic even after at least 30% of an area has recovered and become immune to COVID-19
thelancet.com/journals/lance…

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-19

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 view

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 make

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