What made the UK COVID-19 case count drop?

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'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.

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.

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.

The $R_0$ of Delta is ca. 2x the R0 of the Wuhan strain and this doubles the effect of new immunity on $R_t$. 

In fact, the ONS data gives me that ~7% of Scotland had Delta so that's a reduction in $R_t$ of $R_0$*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 $R_t$ 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.

 

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.

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.

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.

Here's an interesting reasoning: https://covid.joinzoe.com/post/uk-cases-stop-rising 

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.

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?

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 -_-.