According to Google COVID-19 data, something made COVID-19 cases drop after July 21, 2021. To me that event is unexpected given that the measures where in the process of being lifted and I wouldn't expect that to coincide with citizens engaging in more risk avoiding behavior. 

If there's a factor bringing cases down we don't know, understanding what the factor is could be very valuable. Does anybody have a good explanation for why the cases dropped?

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Aug 02, 2021


One factor I've heard discussed is the UK's very high rate of prior immunity (from both infection and vaccination). About 90% of adults in the UK have COVID antibodies (either from vaccination or infection), meaning the fully susceptible population is very small. Obviously breakthrough cases are possible in people with antibodies, especially if they've only got one dose of the AZ vaccine, but R in that population will be much lower. Given the high initial rates of prior immunity, it would be expected that R will drop pretty quickly as COVID "buns though" the remaining susceptible population. 

I don't see how that explains the drastic shift. The UK also had high vaccination rates in the beginning of July.

Sure, the UK had high vaccination rates going into this wave, but the emergence of the delta variant (plus loosening of restrictions) greatly increased COVID transmission. So you'd expect a growth in case numbers. But if there aren't that many fully susceptible people to infect, the case counts wouldn't peak at a high number before turning around because of population immunity.  However, I want to be clear that I think this is just one factor, not the only thing going on. If you play around with SIR model parameters, you can see that you can't get quite as steep a drop in cases as there appears to be in the UK data just by inputting some reasonable values for delta. Changes in the number of tests, weather, behavior, etc. are all potentially playing a role as well, but I don't think we should discount the basic role of immunity. 
R fell from something like 1.4 to 0.7 in a week. If we would be talking about a change from 1.2 to 1.1 in a week basic immunity seems like a reasonable explanation. For the change we are seeing it doesn't seem to be. 
Could this be something of statistical a mirage? (and hopefully this is not too poorly expressed or thought out as it's very much an off the cuff type thought. It's also really just a slightly different statement of the above explanation.) I don't know if this hypothesis comes close to fitting with the reality in the UK but what if one is looking at general, aggregate statistics but the cases are largely in some "unique" sub populations. If there were pockets where previously few people were infected, were taking their time (or were anti-vaccers) and then delta hit those areas. Previously COVID was spreading slowly in such areas. Now it starts spreading quickly. The aggregate measures suddenly turn up but as soon as those sub-populations start to look like the general population you see a very rapid drop in transmission and in the aggregate numbers. Putting it a bit differently, is it maybe one of those devils in the details, but not necessarily a factor that we can really do much with other than note it needs to be kept in mind at times?

I agree that this seems to explain it, but it raises a new question: how did the antibody rate get so high? Is it possible that part of Delta's contagiousness is that it has a lot more carriers who don't get sick?

I actually don't think the high level of antibodies should be such a surprise. I updated my original comment to clarify, but much of that is from vaccination, not from natural infection. Between high rates of vaccination plus historical infections, it's not surprising to me that such a high fraction of adults in the UK have antibodies.
I think I agree. I hadn't realized the UK vaccination rates were so high. In that case I'll lean towards the pockets of unvaccinated reaching herd immunity + shorter incubation period hypothesis.

Unfortunately for this explanation, COVID cases in the UK appear to be rising once again. At this point I'm at a loss for potential explanations of what could have caused the rapid rise, fall, and rise again in cases. 

Andy Jones

Aug 03, 2021


I've been surprised by this too, and my best explanation so far is schools. Evidence in favour is that Scottish school holidays start end-of-June, while English school holidays start middle-of-July, and indeed there looks to be a two-week difference in the peaks for the two nations.

A good test for this will be this week's ONS report. This doesn't have the English turn-around in it yet, but if it is schools then there should be an extremely sharp drop in the school-age rates.

All that said, it's only my best hypothesis. A strong piece of evidence against it is that we haven't seen the same effect in the US, where school holidays started a while ago.

One piece of evidence in favour of this is that the Scottish schools finish earlier than the English schools. Correspondingly, the Scottish case rates started dropping earlier than the English rates.

However the Northern Ireland dates don't appear to match up so nicely - they have an early summer holiday but the case rates peaked roughly when England's did.

So I'm not really sure!

The Netherlands had a similar trajectory. Would be interesting if their holidays matched.
It seems Netherlands summer holidays depend on which region you're in (North, Central, South) each of which is separated by a week (10th, 17th, 24th July respectively). However it seems that no matter which region you look at the peak is pretty close to the 17th.  Possibly with enough mixing between the regions this still kinda works but I feel like it's a strike against the theory.
2Alexander Shmidt3y
I see people wonder about what happened in the Netherlands. And though drop might have been accelerated by school holidays, seems like a lot of people are not aware about what was the reason for the sharp rise on the prior week. From locals' perspective it went like this: the government allowed night clubs to open, requiring patrons to either be [fully? don't remember] vaccinated or show recent negative test. Pretty quickly some vaccinated asymptomatic positives infected a lot of unvaccinated negatives and the thing was spreading like wildfire among 20- and 30-year-olds within a week of jam-packed night clubs. (It also so happened that there was a big music festival during those days.) After a week of this, government called in an emergency restriction for all venues to close at midnight (in effect till today). That seemed to help a lot.

I'm pretty sure it's not schools, unless private schools somehow have a massive impact. The case rates were already dropping on July 21st, which is presumably a couple of days after The Event anyway; the summer holidays for state schools (i.e. the vast majority of children) started on the 25th.

Luke Stebbing

Aug 03, 2021


The best plausible explanation I've seen is that Delta's serial interval might be much shorter, which would mean R is lower than you'd think if you assumed Delta had the same serial interval as older strains. (Roughly speaking, in the time it would take Alpha to infect R individuals, Delta has time to infect R and for each of those individuals to infect another R, leading to R + R^2 infections over the same period.) That makes it easier for behavior changes and increasing population immunity to lower R below 1.

The important thing about this hypothesis is that it multiplies the effect of all other modifications, like schools or heat, so it's at least part of the answer whatever the proximal cause is (which I still think is possibly just this + behavioral changes, but do feel a little underwhelmed without some other factor).


Aug 02, 2021


I'm surprised nobody mentioned seasonality. I hypothesize that the biggest reason is that as you start rolling into August and September the outdoors becomes less frequently too hot, leading to more outdoors activity. Similarly, temperatures that are too cold can drive people indoors as well. 

We see a very similar pattern in the US as well. In the summer, it is typically southern states that get hit hardest with new cases. In the winter, it is the states farther north that see the worst surges per capita.

It's possible that seasonality is a factor, but temperature doesn't seem like it would be it. July and August are very similar temperatures in London, and both are slightly hotter than June. 

9Neel Nanda3y
The thing we care about here is the actual weather in the UK over the last few weeks, not the average climate data. In the last few weeks there's been a bit of a heatwave, and everything has been dry and sunny (at least, in London).
Ah I see, thanks for the info

Aug 04, 2021


The  of Delta is ca. 2x the R0 of the Wuhan strain and this doubles the effect of new immunity on 

In fact, the ONS data gives me that ~7% of Scotland had Delta so that's a reduction in  of *7% = 6*7% = 0.42 just from very recent and sudden natural immunity. 

That's not [edited: forgot to say "not"] enough to explain everything, but there are more factors: 

1) Heterogenous immunity: the first people to become immune are often high-risk people who go to superspreader events etc. 

2) Vaccinations also went up. E.g. if 5% of Scotland got vaccinated in the relevant period, and that gives a 50% protection against being infected or infecting others (conditional on being infected), that's another reduction in  of ca. 6*0.05 =  0.18. 

3) Cases were rising and that usually leads to behavior changes like staying at home, cancelling events, and doing more LFD tests at home.



Aug 05, 2021


This is more of a "hypothesis available to me" than an active prediction, and I don't really know offhand how we'd check it. But the European soccer championship ended on July 11th. England hosted a few of the games, notably including the final and semifinals; I'm not sure how much difference that would have made. (I guess the number of people going to the stadium wouldn't have impacted the numbers too much?) But also, my impression is lots of people went to the pub to watch the games, especially the ones with England and especially the final. I could see that making a big difference.

So, one factor might have been that the Euros were making case counts rise faster than they would have done otherwise, and then they stopped doing that.

Aug 05, 2021


I wish I had recorded my thoughts the week before they removed most of their remaining restrictions.  They were something like "only 20% of adults haven't been vaccinated, they're working down the age distribution, and they're infecting as fast as the winter.  At some point, they just have to run out of people."  I thus vaguely expected things to drop, especially as they had turned around before.

One, they're just running out of susceptibles, two, a shorter serial interval means that everything up and down happens faster and the R value is likely not as higher as it would be if you did not take this into account, and three... a good bet has been to expect authorities to be wrong.


Aug 03, 2021


In the UK the Delta variant seems to have followed a similar curve to India: sharp increase followed by sharp fall. The Indian peak was around the begining of May, the UK around the 20th of July. ONS estmates 91.9% of adult UK population have antibodies (both figures are current not at start of curve). In India it's about 67% of population has antibodies. It is not known why the steep fall in cases happened in India. If the patterns are as similar as they appear we can, perhaps, rule out schools and other factors not common to the 2 countries.

It is not known why the steep fall in cases happened in India. 

It coincided in India with rollout of prophylactic ivermectin. While the studies we have for prophylactic ivermectin aren't well powered, they suggest that prophylactic ivermectin reduces COVID-19 infection by an average 86% (95% confidence interval 79%–91%) which works well to explain what we see in India. 

If the pattern in India was very much the same as in the UK, where so far as I know there was no prophylactic ivermectin at all, that makes it less likely that the ivermectin was an important part of the rapid turnaround in India. Do "the studies we have" include that big one that turned out to be probably completely fraudulent?
The study in your link is about using ivermectin for treatment, while this is about ivermectin for prophylaxis so no it doesn't include it.
Aha, OK, then indeed that wouldn't be relevant. Which would be good news for the prospects for prophylactic ivermectin, I guess. (My understanding is that the Serious Medical Establishment mostly reckons that what ivermectin studies there are are too weak to base anything much on; on the one hand, the Serious Medical Establishment seems to have trouble distinguishing "insufficient evidence of the best kinds" from "no evidence at all" from "therefore this treatment is probably harmful on net"; on the other, my impression is that things that look like "insufficient evidence of the best kinds" turn out false much more often than one would naively expect, presumably because of publication biases and the like, so the Serious Medical Establishment's processes are more reasonable than they may appear.)

Alexander Shmidt

Aug 04, 2021


Here's an interesting reasoning: 

As I understand it, it goes something like: the list of symptoms which are required in UK for you to be able to get a free test is pretty much limited to cough or loss of smell or taste, but Delta has a different symptoms profile, so the more people get Delta the less the proportion of those who gets tested.


This wouldn't explain the recent reduction in R because Delta has already been dominant for a while.

Daniel S

Aug 02, 2021


I'm interested in predictions for where in cases/day this rapid decline stops (or steadies out for the medium term). I think different underlying causes for the drop yield different answers.

If "we've reached a vax+infection herd immunity" is to be believed, then we'd expect cases to bottom out pretty low barring any new variants/immune escape yes?

Most other causes yield more depressing answers. If this is behavioral changes or lack of testing I'd expect very little in terms of getting to low absolute case levels. Seasonality perhaps?

The entire turnaround just seems very sharp, which leaves me confused (and looking for a better model). I'm not sure any of the offered explanations sound very convincing to me. This pattern occurring in places besides the UK seems to rule out region-specific explanations like "Freedom day". I think that case levels over the next couple weeks in the UK and other countries following similar trajectories should give some additional evidence?

Zac Hatfield-Dodds

Aug 02, 2021


Based on this article, I'd guess mostly reduced testing following "freedom day":

So, what could be behind the drop in new infections, and is it too early to be celebrating? The Office of National Statistics (ONS) weekly infection survey ... showed the estimated prevalence of infections in England had actually risen from 1 in 75 people to 1 in 65 in the week to July 24.

The ONS survey aims to estimate infection numbers in the community beyond people who have been tested, and gives an estimate of COVID-19's prevalence that is unaffected by fluctuations in people putting their hands up to be tested.

The good news in this is that with the ~88% vaccination rate reducing the death rate in vulnerable and elderly populations, there won't be anywhere near as many casualties as a 'let it rip' approach to herd immunity would have seen before. And at some point the UK will run out of unvaccinated uninfected people; hopefully before sharing another variant with the rest of us -_-.

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I disagree. Daily tests have only gone down slightly, which is to be expected if less people are getting infected (

And crucially, it's not only total positive tests per day that has been dropping;  the share of positive tests has also been going down significantly:

So I don't think your theory passes the sniff test. 
I'm personally very confused about the drop in cases, and none of the theories I've heard seem likely. 

This seems highly unlikely. There hasn't been a significant drop in testing, and Scotland (which saw an earlier peak) has also seen a drop in hospitalizations, which are much harder to fake.

Isn't "prevalence of infections ... [rising] ... in the week to July 24" completely consistent with the cases peaking on the 16th? Given people are infected for ~10 days, and cases were higher in the week to 24th than the equivalent period 10 days earlier.

Calculating the 10-day rolling average of cases* in the UK (from here) I think that the prevalence should have increased by ~10%. The prevalence increasing by 15% obviously suggests there was some affect of fewer people being tested but also cases are definitely declining. (Usually attributed to schools d... (read more)

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alexrjl collected a decent number of explanations here.

I think it's worth considering the wider story: similar drops were observed in the Netherlands and Portugal.

The Netherlands reimposed restrictions, whereas the UK opened up. I think the UK has simply overshot herd immunity and this leads to a sudden drop.

While Germany didn't had drop in cases R also dropped for some reason.

One hypothesis I have for which I unfortunately haven't found good data to check is increased humidity as a common factor that happened cross country in Europe.